FWIW, In a recent issue of one or the other gun rags, Mas Ayoob comments that the cylinder of the LCR is slightly smaller in diameter than the J; at least so far as speed loaders are concerned.
That said, it can't be that much practical difference. When I finally had a chance to fondle one at my local store, it struck me as very J–only with the fat grips you mention, again FWEIW.