The "hotness" of radishes results from the length of time they have grown rather than from their size. The radishes either grew too slowly or are too old. Radishes like cool weather, but it needs to be warm and wet enough for them to fill out before the weather heats up.
2. Why Do My Radishes Crack Open?
3. Why Do My Radishes Get Tough and Woody?
4. All I Have are Green Tops. Why Aren’t the Radish Bulbs Forming?
Planting too thickly and not thinning to about 1-2" between plants will also cause radishes not to develop. They are too crowded and don’t feel they have room to plump up, so again, they will go to seed.
Another cause of under development in radishes is not enough sunlight. Radishes can handle a little shade, especially if the temperatures are creeping up, but they need several hours of direct sun to fully develop.
Of course, if you want to avoid this problem all together, you can simply grow edible podded radishes - radishes grown for their crunchy, tangy seed pods, like Rat Tail. They love hot weather, have few pest problems and never form bulbs.