> Basically, as others have said, there is an evolutionary reason plants do not naturally grow this big (they'd kill themselves off in a few generations).

I don't know that this is solid reasoning. Certainly there is some limit eventually imposed by soil nutrient content but there could also be other reasons plants don't grow large fruit. A big one being that plants which grow their fruit to spread seeds reach efficiency when there is enough fruit to attract a spreading organism and no more.

In fact, basically all the plants we consume were already unusually large even before the green revolution. Humans had selected or hybridized the popular cultivars to an extreme already.

Again, I'm not saying it's impossible that this size of plant crosses some threshold for the soil, it's just not safe to assume it does either.