AFAIK .Obj won't support animation anyway, but regardless - the reason why the file is so big is that it'll be human readable - so saved as text pretty much. Where a binary file for example might use 4 bytes to store a float, an .Obj file will use 8, and there will be extras like tabs and stuff that bloat it even more.
If you can find it, CharacterFX is pretty good - easy to use and exports straight to .X files that are perfectly compatible. It would be quite tedious to use with a very high poly model as it is quite old, but I find it so much quicker to use than Blender, or any other animation system for that matter.
The code is dark and full of errors