|How exactly are they too big? Is it just that they don't fit under clothing? Or do they gap a lot at the legs or waist? At 21 lbs he's at the very beginning of the weight range but still within it. My son started wearing the larges when he was about 17 lbs (and not chunky at all, very tall but very lean). They were bigger than they needed to be but it didn't cause any problems like leaks or anything like that. Is this the first time you've used cloth diapers? If it is it's often a bit of a difficult transition when you're used to the extreme trimness of a disposable.
The smalls would probably still technically fit him but I personally don't think they'd be a good buy, they'd be outgrown much too quickly. That said, as long as the diapers aren't washed or worn then it's not too late to return them. ME accepts returns up to 6 months later as long as they're in resaleable condition. You could always sell them and recoup a good portion of your money if you find the fit really just doesn't work for you at all. The One-Size (without a liner) do fit trimmer than the Lg Sandy's (although usually bulkier than the smalls) so that might be something worth trying...