Lack of rain ???

What variety? If you want large dessert grapes from any type of grape, you have to thin the fruit from an early stage, using nail scissors to thin out the fruit in the bunch.
Grapes grow well in quite dry conditions so I'd be surprised if lack of moisture is the cause if the vine is well established.
Many wine varieties don't start to swell until the autumn so this could be the case depending on what you have.

That could be it, Rotoman, or the the fruit clusters weren't spaced out enough at flowering--you only want one cluster every 50-70 cm to get full-sized fruit. Some varieties don't get big fruit unles they are sprayed with giberellic acid. 'Thompson's Seedless' is notorious for that.

TT is right, lack of rain means slow growth and the vines are not getting enough food, also you need to thin your bunches and not let the vines carry too many bunches. Damping down around them will also help.

Sorry Bertie you came in before I came on, similar minds. But we are suffering the worst drought since the early 20's over here.

Must admit I've never grown grapes ... but my response was because it has been such an exceptionally dry summer in so many parts of the UK. :o)

Sorry, Rotoman, Bertie must have posted while I was writing. What I meant was that drought was a likely cause, like Terratoonie said.