I leave some on a second year vine if it's big enough. So vine size is the key in my opinion. As with all young fruit vines/trees, I thin extra heavily. So how big is your vine? What is the caliper of the trunk? How big and long are the canes and how many did you leave? How many sq ft would you expect the vine to cover this year? Is it in full sun? These are the factors I'd consider.

Remy: Thompson seedless are naturally small berries. What you are seeing is somewhere near the normal berry size of that cultivar. Commercial growers achieve larger berries by spraying gibberellic acid to first thin the fruit off each cluster and then to plump up the remaining berries. They also use other tricks like girdling the trunk. You could do this. It's not that hard. But it doesn't increase the flavor or sweetness and may do the opposite. Doing a better job of pruning, thinning, and other cultural tasks can increase berry size somewhat if they are limiting factors. Commercially berry size increase is used only for Thompsons destined for table grape use or other special applications like Jumbo raisins. The normal raisins are grown with small berries just like yours. So the vast majority of Thompsons are grown like yours with small berries.

And lastly, this is actually on my Healthier vine. >> I think I wasn't watering them enough when it started warming up and only really started watering them every two days about a month ago so this could be an older leaf from that. But just putting this here in case it can help>

Pretty darn impressive and a clever way to grow it. We have some wild grapes here (muscadines) that are about twice as tall and twice as thick as that. I'd guess ~80 feet up into crowns of forest trees.