I watched a coaching video just now, and the coach was talking about a flop spot: COvMP 3BPot (mp rfi, co 3b, call) hero w/ AdKd

board comes down Ts 7h Qd, villain Xs

the coach said something to the effect of, "i think pio would favor a small size here, a smaller size here allows us fit more bluffs in."

This statement is confusing to me because my understanding has always been that larger sizings allow us to add more bluffs (for example overbetting river, and the toy games prove this). However it makes sense that we could fit more bluffs in on earlier streets using a smaller sizing, as kind of a 'mergy' play...although I am confused. Can someone clarify how small sizing OTF impact the amt of represented value we can proceed with? Do small sizings on early streets allow us to fit more bluffs in because we dont really have bluffs OTF: just speculative/drawing EQ?

I wouldn't guess that we'd want to use a small sizing here, but a small sizing also allows us to value bet weaker holdings, which can be balanced with more bluffs. The toy game your thinking of has polarized ranges where the size of the value range stays the same as the bet size increases.