Can anyone give me some ideas on how to stay warm without bulking up too much?

Refried beans the night before?

Seriously though, I'm still trying to figure out the same thing.  The little disposable handwarmers my wife uses skiing look tempting.  Thinking of sticking one in each boot to start with.

Silk base layer, mid/heavy fleece, topped off with lightweight GoreTex outer layer.  Gaiter around neck and good head covering...hands inside a belt type hand warmer with hot packs...good, quality insulated boots over wool socks....Arctic Shield boot covers over the boots.
Another helpful tip is to place the hand warmers under your arms, over your kidneys and around your neck..
Also, keep your body off the ground, it will suck the heat right out of you....plus, don't put anything but the sole of your boot against the ground.  Most ground hunters get cold quick because their legs and butts are in contact with the ground.
Hope this helps.

Drawing on my Marine Corps cold weather training here...

Calories- impossible to stay warm without some extra energy to burn.

Loose fitting layers- I was out tonight for a few hours just had wool long johns, wool trousers, sweater and an Asbel shirt.  Boiled wool hat on my head.

Feet- silk sock liners and a thick pair of wool socks over them, and my Maine hunting shoes to keep out the wet tonight.  I was just right.  Granted, it was only barely below freezing in my location so not too cold.

Hope this helps a little.
Tom

Ken has it figured out, for sure.  Avoid cotton next to the skin.  A silk base layer is great.

Layers, wool, stay dry, keep moving and adjusting the amounts of layers.  Believe it or not, it is that simple.

Jarhead training rules. Phrog nailed it.  Hat, neck scarf are musts and chocolate candy the instant you shiver. Bakked potato in foil in the pocket.  Keeps hands warm, them provide nurishment. Layer up, and in windy conditions make sure you have a windproof layer, does not have to be the outer layer.  Think about insulation between the metal tree stand and your boots. I retired from the Mountain Warfare Training Center and got to have the best equipment and training available.  Those guys go all over the world reviewing techniques and equipment.

I would be in agreement with all of you with the exception of one fact that bubbaPa was looking for and that was the "Bulk"
Issue especially for archery. And clothes getting in the way of string.  I've gone like others are saying.
Base SILK
MID: 240 gm wool (I like first lite) cause if I pull my outer layer off I'm still camo.
OUTER: something tighter to keep the bulk down wind and somewhat water resistant.  The newer synthetics are ridiculous quiet too.
If not hunting I do what PHROG does and stay loose layered.

I've found that a merino wool liner sock, then a wool boot sock, and hot pads on my toes will keep me warm with 800g boots in weather down to 0*F.  For the rest of my body I wear insulated pants, jeans, then thermals.  My upper body is covered with a thermal coat, sweatshirt, then a t-shirt.  Keeping the sweat away from your skin is important to keep from getting cold.

Yep, good layers.  I also found a good wool vest  is huge for adding warmth to core without adding bulk.  Use a down vest if you really want to make some heat.

Primaloft and merino wool. Wind proof outer layer. Keep your neck head and wrists warm, that's were you lose the most heat. High fat content food. Mountaineers eat salami dark choc peanut butter and olive oil

Agree with above.  Good silk base layer, wool and a good hat.

I wear and recommend Terramar long underwear from Sierra.

What about Zippo "heater", the kind you can put inside your jacket, that stay warm for hours, and are reusable, I have a friend that swear by them! Any one uses them?

Gray Wolf Woolens Vest is the best piece of clothing I own and will eliminate those bulky arms. I wear Ulffrotte' 400 gr wool long johns, then a heavy loose wool sweater, then the vest over that and I am toasty on top all the way down to 10-15 degrees as long as the wind is not heavy, then my arms would get cold.

On windy days I substitute the wool sweater mid layer with my Cabelas wind proof extra heavy lined sweater but it has to be 15 degrees and at least a 10 mph wind to break that out.

Agree with the above....and can't add much to the actual gear since gear has been covered pretty well.  With that in mind I'll add that you need to do everything you can to avoid sweating while walking to your stand or between stops while still hunting.  If I am hunting from a stand I carry my outer layer or even two outer layers in my pack. For my hat while walking I wear a light weight stocking cap and switch to my warm stocking cap when I get settled in.  Feet are a big issue too...it's hard to wear enough thick socks inside thinsulate boots and not have your feet sweat when walking.  Your feet will stay warmer in thinner socks if your boots aren't tight.  If you can't wiggle your toes and flex your feet your boots are too tight.  Tight boots restrict blood flow, don't allow for insulating air space, and actually conduct the cold from the outside in.  Another big tip that will help is to WALK slow.  Keep your heart and breathing rate low and your body won't build heat and cause you to sweat.  I like to leave my truck early enough that I can walk very slowly if it's dark or still hunt to my stand if it's daytime.  Not only do you not break a sweat as easily you might also stalk within bow range and release an arrow if it's daytime.

Quality clothing, in layers. I walk in with very little clothes on, once settled in my stand I add layers as needed. If you get sweaty, than you will later get cold. Getting damp from sweat will make you miserable. The "hothands" chemical handwarmers are good too. I use boot blankets vs heavy insulated boots. The insulated boots just sweat my feet walking in, than freeze them later. I use uninsulated boots, than ad boot blakets over my shoes after settling in. A hot hands pouch in these will keeop you toasty. Keep feet, hands, and head warm, you will do fine. vests are good too, keeping the body core warm, but giving good mobility for bowshooting.

David Yukon, I used a Zippo hand warmer while hunting and ice fishing when I was younger. They work great, and I've been thinking of getting another one.

Thermacare back warmers.

Heater Body Suit

Hunt in Hawaii .....