Most probably because they are a 'long' variety....French Breakfast maybe?
Some of my early sowing of radishes (Cherrybelle) have not swelled into round radishes and are now running to flower, whereas a sowing 4 weeks later has produced lovely crisp round radishes - I put the difference down to the slow cold season.
My early sowings of french breakfast and cherrybelle radish were small and didn't swell but later one's have produced a better harvest, I agree with Dovefromabove .
What variety have you sown, mooli are a radish and there's also a black variety, both grow to about the size of a carrot/parsnip. Mooli are better sown in spring and autumn, I've just finished picking the last of my mooli and planted out black radish, it forked last year, I'm hoping for a better result this time.