Watermelon Problems

There are a variety of things that could cause your watermelon to be small. It could be either underwatered, undernourished, or possibly too mature when you planted them (if you planted seedlings rather than seeds). Vegetables need soil rich in organic matter, which can usually be found in a high quality compost. They also need the right food sources, fertilizers, to meet their needs, plus ample water.
Or, you may have planted a small “icebox” size watermelon in the first place. Varieties that mature quickly (what we need in our short growing season) tend to be smaller.
For maximum size, allow just one or two melons to mature per vine and remove the rest. That way the plant will put all its energy into just those few fruit.