Very odd.
Although, if you've got the right version of the mod, the planets are actually normal size:
It is your perspective that is tinier
As I recall, the first time I tried to make the jumbo system, I tried to make the planets bigger, and it choked on the huge map...
Try the supersize P&Nism mod. I'm sure that one works, at least for me.
I'll look into the supersized stock this afternoon.