That is curious Aqui. Mine grew to the size of tennis balls and yet were yummy and crunchy and jiucy all the way through - just hoping second sowing will be ready to start harvesting next week.
Mine were nibbled by slugs, but they only seemed to skim the outer surface, and that was it. To be honest, you couldn't tell unless you looked real hard, or caught one in action. My glorious Essex soil is also like a cobbled road, but only one was mishapen, looked like a peach, due to it growing around a stone. I think water and speed of growth are the key factors with radish. Get 'em sowed, watered, germinated, watered, thinned, watered, picked all as quickly as possible!
Did you sow a second and third batch so you can keep picking?