What causes my Tendersweet yellow watermelon to grow large, get ripe, but be hollow in center?
This sounds like hollow heart of watermelons, of which the cause is not known for certain but normally related to weather conditions in addition to poor pollination. Uneven moisture levels and too much fertilizer may cause hollowed out watermelons. Seedless types are more prone to this disorder as well. Other than maintaining even moisture levels and being careful not to over fertilize, there is really little else you can do as you cannot control the weather and if there is lots of rain, pollinators will not be able to do their job. Hand pollination, however, may help with this. Here is more info on that: https://www.gardeningknowhow.com/edible/fruits/melons/hand-pollinating-melons.htm