Lots of great ideas… soil, magic difference, cutting, shrinking in size (but would the beans produced be super tiny then? ). The writers could easily have written something in with a line of dialogue. It’s kind of a plot hole, that I think came about because the writers wanted a reason there were no extra beans back when Rumpel and Bae used one. If the giants (like dwarves) have always had this duty to grow beans, then why weren’t they doing it back then? So whatever explanation they choose for why the beans grow fast in SB, also could have been used to explain why the giants didn’t have more beans back then (soil was rotten, magic rules, didn’t have any cuttings so started from regular beans, giant beans take forever to grow…).
“If I had a world of my own, everything would be nonsense. Nothing would be what it is, because everything would be what it isn't. And contrary wise, what is, it wouldn't be. And what it wouldn't be, it would. You see?” -- Lewis Carroll, Alice's Adventures in Wonderland & Through the Looking-Glass