What is a "big pawn"? I've seen the term "big pawn" come up a few times (such as here and here), but I can't seem to trace down an exact definition. What is it?
A piece (often a bad bishop, but in your second example a queen) which is doing nothing more than protecting a few squares is often called a big pawn. To be honest, I don't find your second example a real case of a 'big pawn', because the queen can easily move away and do something useful. And, as Dag Oskar Madsen mentions, in the first example the pawn breakthrough is big, not the pawn itself.

In the position below, the white bishop is a typical example of a big pawn, because of Black's dominance on the dark squares:

[FEN "r4r2/5pkp/1q1pb1p1/2p1n3/2P1P3/1P3P2/2BN2PP/1R1Q1RK1 b - - 0 1"]