Thanks for the message, really good to hear.

I will try to explain it from a theory perspective and then an exploitable perspective.

Theory, yes we do occasionally bet here to protect marginal showdown/value hands, but generally it wont be bottom pair, we would do this more with 2nd pair and when having a little more equity in the hand, but also being able to bet call a raise and also being able get called by reasonable amount of hands which we beat. For example on this flop a hand like J8,98, potentially 86 (but I imagine this mostly checks) especially with a spade. As you can see a hand like 74o doesn't fit this criteria, its too weak to bet flop and we really don't want to get blown off our equity.

Now from an exploitable stance, population tend to craft their range on the flop much differently to how Pio would. From my experience players will over stab on the flop with a bunch of their range, consisting of bluffs, but also stronger value, but will check back a lot of their marginal showdown type hands (2nd and 3rd pairs). Now on the turn again their range is crafted differently to Pio given their flop strategy and so when villain bets the turn for this sizing I think there it too much 2nd pairs and strong 3rd pairs relative to their overall range hence the fold.

74o in theory is a call, but its not making a ton, so as an exploit folding out the bottom of the range is going to work out better vs how I perceive population to play.

If villain had bet 25% or 40% on the flop I wouldn't be folding based on how I think their range is constructed. I hope that answered your question a little better. Obviously vs some players I would be looking to call 74o on the turn, but overall I think it is going to work well as a fold.

I would classify a 2x as small, ~3x as large and ~4x as very large. Generally the way I approach these spots at chip EV is I open a smaller size which allows me to play more hands and I think this is the way you should do it if you think you have a post flop edge.