I agree with Italophile in that beetroot doesn't always develop at the same rate, you can get huge tennis balls sizes along side golf ball sizes.
..and regards a crop for this year it has been the worst ever for me, beetroot usually grows for fun in the garden, I was still eating pickled beetroot until May from last year. Planted two varieties this time - Detroit 2 and Burpees Golden along side each other both had the same growing conditions and they've developed the same as yours, so did a MD last weekend and lifted the lot, had more BG than D2 but only enough for a meal. Sowed some more seeds of both varieties under cover in the veg plot and they are already showing through.
I'd put the poor growth down to the weather - too much rain and not enough heat or sun. Try sowing again, depending where you are in the country it's not too late for a late crop, that's if we get some heat and sun. What do you have to lose, you may get golf ball sized beetroot, a good size to pickle