If they were the same variety then "I guess the answer lies in the soil" as Sir Percy Thrower used to say. A hign notrogen feed will give a lot of top growth BUT I guess the most likely answer is a different variety or even a late planting.

The common varieties are white "Snowball" which is Apple sized and small is Primera F1 Table tennis ball sized. Many inbetween. Some are grown purely for their leaves.....Then they are planted closely and NOT thinned. Turnips should grow quickly, 40 days and be harvested when not fully grown for best flavour. Keep well watered and sow in good manured soil.

you can laugh if you want to--but i've found that planting by the moon causes these variables. i tried it one year on purpose, and wrong sign produced only tops-no turnips--while right sign produced very little leaves--and big turnips.

and--yes, there is a variety that is grown only for tops. i just remembered, years ago, i planted without reading the package. read it--after having only tops.

Seven Tops is the variety that I know of that is grown for the greens only.