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This happens for the same reason as other root crops, like beets. That being said, the following article should help: https://www.gardeningknowhow.com/edible/vegetables/beets/beets-small-or-deformed.htm

This is typically caused by the soil being too warm. This article will help you correct this:
https://www.gardeningknowhow.com/edible/vegetables/radish/radishes-bolting.htm

Too much nitrogen is what happened. When plants have too much nitrogen, they get very leafy but have little fruit and roots. Give the plant some bone meal or something else containing phosphorous to rebalance the phosphorous and nitrogen in the soil. Here is more information: https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/understanding-nitrogen-requirements-for-plants.htm, https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/phosphorus-plant-growth.htm

There are several kinds of pests, including caterpillars, that feed on the tops. Without seeing the bug, I cannot ID it. But, the standard organic treatment for most pests is neem oil and may work on caterpillars too. In addition, Bacillus thuringiensis, also known as Bt, effective for caterpillars. As long as the bulb and roots are still intact and healthy, the radish will continue to grow.

A few things to consider: too much nitrogen, not enough light, too much heat (check seed package), not thinning the sprouts. Here are some tips on radish growing: https://www.gardeningknowhow.com/edible/vegetables/radish/tips-how-plant-radish.htm

A neem oil spray should fix your little friend right up. https://www.gardeningknowhow.com/plant-problems/pests/pesticides/neem-oil-uses.htm