Think of it this way (as an example):
Your tank is in a hot room and you're water evaporates at 1 gallon per week.
If you have a 10 gallon tank, that 1 gallon per week makes a HUGE change in the chemistry of your water. If you have a 75 gallon tank, that 1 gallon isn't nearly as harmful.
The bigger mass you have, the easier it is to keep your elements under control.
Hope that helps!