tell that to the hordes of small bream and tench I had on the country park fishing 18+15mm snowmen pop ups 3 inches off the deck with size 4s!! if they are ravenous then they will try their luck. But on average yeah, they do pick up the bigger fish and avoid the small stuff.
Yeah, I have dropped down to 15mm pops for the estate and fish them tight as possible to the deck most of the time however after a bit of maths...
53 fish landed since June 16th, 2 on bread crust and 3 on snowman baits... leaves 48 on pop ups
Total combined weight of these was 855.07lb with an average of 17.12lb approx. which sounds about right... 48 fish including 20 fish over 20lb, 5 of those being over 25lb, and one of those breaking the 30lb barrier.
Coincidentally all the fish on snowman baits and off the surface were below 15lb... or maybe not?
Luck does come in to it as I said above about the tench and bream! But I do think pop-ups, blatant high ones do pick off bigger fish most of the time.
Main reason to use pop-ups is that 90% of those gravel spots you find will have a bit of something on them, silt, silkweed, etc. and it's why I use pop ups just over hook height, to keep the hook point clear of it all