I’m growing several varieties of grapes in 5 gallon buckets and 7 gallon pots, a couple of which have fruited and are fruiting. The fruits always mature small. Is there any particular reason for this? What can I do to get bigger grapes? For reference, the varieties that have fruited are Niagara and Chambourcin (thank you @IL847 ). I’m also growing Concord, Cotton Candy, Exotica and Fredonia. I’ve also lost track of most of their identities. 
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Fruit trees in pots also usually have small fruit. Thinning more should help them. Although in my experience the fruit is still small but maybe I’m not thinning enough. Grapes could, and likely should, be thinned as well. Reduce cluster number and perhaps cut off the cluster tip.
I just planted some grapes in 15 gal pots. I’ll likely leave no more than 4-5 clusters. Just 1-2 the first crop.
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if it is chambourcin, it should get darker and a little bigger. It looks to me your vine is unable to support so many fruits so it struggles to color up. I had my grapes over set one year and that was the color my grapes had when I harvested them.
I late learned to prune the shoots and prune the clusters. My grapes were back to normal the next year
BTW. Chambourcin has European grapes leafs. Hope this help narrowed down the identity of cultivars
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Irrelevant to your questions, but is Cotton Candy available in some parts of the world? I was under the impression it was not available anywhere for home growers.
I was about to ask the same thing.
I assume the grapes pictured are Niagara, a white grape, so it won’t color up. The size looks fairly typical to me. The grapes in grocery stores are usually pumped up with gibberellic acid, which makes them much bigger than most “backyard” grapes.
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That happened with my sugar apples. I was hoping it wouldn’t be the case with grapes, I’ve seen people grow grapes in buckets and other containers. 
Actually it seems to be Chambourcin. The Niagara’s I’ve fruited matured paler and green, there’s definitely some anthocyanins on those grapes.
I’ll correct myself… “allegedly” Cotton Candy. That’s what I bought the cuttings as on eBay on November 2021, from vendor vasyl_svet.
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Gotcha! I have seen listings now and then.