Seedlings are programed to stretch to beat surrounding vegetation. They should stop somewhere between the first and third set of true leaves. Full sun and wind help big time. You can do little seedling pushups from side to side too, just don't break it. If your starting them indoors then put the light on the side and rotate the plant 180 after it grows to the light so it has to bend back the other way. You could also just plant it deeper which will also cause larger root mass.