Simple Physics
Newtons Third Law
"For every action there is an equal and opposite reaction"
Although these are just numbers I've tossed out, they should explain it:
In other words you weight 200 lbs and are lifting 225 lbs with ~225 lbs of force as you press up, the weight presses back for ~225 lbs of force. You are on a bench which pushes that back up at you so on and so forth.
Now increase your weight to 210 lbs from some good old salt and water (water weight/retention) and suddenly 225 lbs you push against the weight being pushed back at you is working against a larger object(you) so the downward force is less effective and thus your upward force is relatively more effective.
Imagine even in Space where Newtons laws really are perfect.
No gravity. You can easily push another person your size away from you, but you go backward with just as much force as they go backwards. Now push someone half your size away and you go back half as much as they do. Now imagine out in space, still no gravity and you are outside the space shuttle and push the shuttle. Gravity is not preventing you from moving it and your force will indead move it a few micrometers, but for the most part you will go flying backward.
Now drink enough water to make you bigger than the space shuttle. Same muscle mass though and suddenly the space shuttle moves just as far as you do ;-)
SIMPLE PHYSICS