I am fairly new to bow hunting (2nd year) and I was wondering how do you decide which weight broad head to shoot, 100, 125, etc. and why do you shoot that weight?

Does the 125 slow your arrow speed compared to the 100 and do you need to shoot different weight arrows based upon the weight of your broad head?

The weight of the broadhead should be decided after you've choosen your bow and the arrow you intend to shoot.IMO There is a great article on this home page which talks about Front Of Center.FOC is expressed in a percent and basically just tells you where the balance point of the arrow is. FOC will determine how your arrow flies after being shot. FOC between 7% and 10% will provide you with the flattest possiable trajectory and FOC generally range between 7% and 18%.
Here is the link to the article. I'm sure it willl answer a lot of question you may have. I know it did for me.
http://www.bowhunting.com/everythingarrows.asp?page=5

I hope this helps some

Overall, broadhead weight and size depend on your bow poundage, your bows IBO speed, and the type of game you are hunting. I generally use a thunderhead 125 with my 60# bow because it increases my penetration. But that is for my bow, your bow may be different. Of course size matters if you are going hunting for Elk or for Antelope. You wouldn't use a 85 gr. broadhead on a bull moose nor would you use a 145 gr. on small varmints. I hope I helped ya some.

Bow draw weight:
45-55 = 100 gr.
55-75 = 100-125 gr.
traditional bow: 145+

Taylor

I agree with the FOC and spine considerations.

But I also would have NO hesitation flinging an 85 grn Thunderhead or 75 grn muzzy at a Moose or any other animal NA can throw at me.

The broadhead is only one factor in weight/KE/penetration.

Shoot what is comfortable to you, i shoot the same weight in field tips and broad heads. works good for me, but i shoot machanicales as well