Watermelons Too Small
My watermelons last year were small and someone told me that I need to thin them out. I wasn't sure if I needed to thin the buds or wait until the watermelons have started to form.
Watermelons are heavy feeders, so make sure the area in which you plant them is rich in organic materials. Normally, you only thin them if you are looking to grow a big melon. Do it after the melons have formed. Keep the strongest, largest one. Water deeply and side dress with compost.
This article should be helpful: