The reason for gru being so strong is because the second letter of his name 'r' is the

The reason for gru being so strong is because the second letter of his name 'r' is the 3rd letter of shrek so therefore gru can use a 3rd of shreks power.
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Or maybe his shrink ray had an anti gravity field it places on the items it shrinks so that it is easier to transport what it shrinks.

Consider that the moon shrinking down but not changing mass would mean that it's gravitational field would increase. Why is nothing in the surroundings being pulled or crunched on this Field Force? Simple, sheer Gru willpower.