Every now and again you'll plant watermelons and yes you do get melon, but they're squeamishly small and stunted. There are a number of reason watermelons fail to grow to their anticipated size and at times even fail to ripen at all.
The first and foremost factor to consider is did you wait long enough ? Watermelons have differring maturity dates, anywhere from 70 - 130 days or more dependant on the variety and environmental factors. Check the seed packet for the maturity date.
The second factor to consider assuming they have reached their maturity as per the cultivar, is how big the variety you planted is supposed to be. Are you sure you didn't plant a personal or mini style watermelon ?
Transplant damage. If you started your watermelons from transplants as opposed to direct seeding, there's a possibility you damaged the roots when doing so.
Damaged roots canít always take up sufficient nutrients to support the hefty growth rate of watermelons. Deep cultivation too close to the plant can also cause root damage.
Pollination is needed to produce quality watermelons. If weather was poor at pollination time, chances are your vines did not get as much traffic from bees and butterflies as they should have. Poorly pollinated plants will produce fewer watermelons, smaller melons and substandard ones. Inter planting a diversity of plants that attract bees in the vicinity helps to attract pollinators and reduce the risk of inadequate pollination.
The primary reason for small melons is temperature. Watermelons thrive in the dog days of summer, they are heat lovers. A minimum of 60 degrees, but preferably more than a tad higher is needed. Lower temperatures leads to a slower plant growth and ultimately stunted melons that are usually substandard in other mannerisms as well. Best day time temperatures for Watermelon is 80 to 90 degrees, night time no lower than 60 degrees F.