Dog's Bathroom Habits

I am scientifically astir tonight over a very interesting subject that is fraught with mystery. Dogs taking their potty breaks.

Sometimes I take my dog outside in the yard .... and sometimes I take her to the park. If I take her in the yard her output -- 1 and 2 -- is at one level. But if I take her to the park it is generally twice that or more.

This leads to the obvious question. Are dogs just storing it up and still feeling comfortable while walking around inside? Doesn't it stand to reason that when you go out -- whether in the yard or at the park -- the output would have to be pretty much the same?

I need a good website to research this stuff at.