Hi Smuff & welcome.
I grow outdoor grapes over an arch & my grapes are quite small but used to be very tiny until I started to thin them out a bit. You're supposed to cut off any small bunches & leave just one decent bunch on each fruiting spur & also trim out a few individual grapes from each bunch using scissors (try not to touch the grapes as it can damage them), I usually just cut out any odd tiny grapes from each bunch.
