I don't know if reptiles undergo a pubescent stage, but id assume that because they must age to reach sexual maturity, they (even if its slow and ongoing) can in essence be "prepubescent" in what applies to my argument here:
Prepubescent muscle has higher muscle endurance while "adult" muscle has higher maximum force per mm/cross section.
What happens when you "stretch" a muscle (which is happening when attempting to "pop" a snake) is that there is a receptor on the muscle which senses that it is being stretched and will cause the muscle to contract if it is being stretched farther than it "should be" (depends on many factors)
In an adult (basing this off my understanding of human muscle and pre/post pubescent models) the stretch reflex will be much stronger and therefore harder to "pop"
So an 8 foot snake that isnt "an adult" yet could actually be "weaker" in that small muscle that holds their gonads inside them in comparison to a smaller snake that has reached full maturity.
this is following the same principle as to why a 130 lb adult can produce much more force per muscle size than a 130 lb untrained 12 year old, though they have the same "size" muscles
hope that wasnt too confusing I tried to relate it to snakes
(also, upon a little research, testosterone has opposite "growth" effects as it does in mammals for reptiles, which is why many males are smaller than females)
Threaded Mode