Again, that depends on the writer's instruction, as well as who is coding it; but overall, according to Fenoxo, size locking is not an intended mechanic (the size difference should be the core mechanic instead). So if a writer ever comes in to fill those particular details, then those scenes can be expanded to support oversized genitalia--I recall it happening before, if not for that property specifically, then for other characteristics, so this isn't an impossibility. It just needs to be written.
From what I can see in the code, when a Behemoth is generated upon encounter, those that have female parts should have a vagina that can fit a 5 foot-long Behemoth fully (+10% more of that size and an additional random volume value between 0 and 200 cubic inches); and an Alpha (both male and herm) will have a similar anal capacity (herm will have between 0 and 150 for their random value). This should be a bonus capacity on top of whatever their default vaginal/anal capacities are (around 150 cubic inches, I think?).
So if your character's genitals aren't fitting then they are probably over that size (somewhere between about 5000 and 9000 cubic inches of volume, roughly). Keep in mind that the Behemoths are generated with randomized values, so not all of them will have the highest volume number (but they should all fit that 5-foot behemoth penis minimum). I won't know how far the mismatch is unless I know your character's sizes.
