We know Kepler results are heavily biased for (1) systems edge-on to us (2) with big planets (3) very close to their star.
The only conclusion to draw is that planets that are all clustered close to their star tend to similar sizes. Then only Mercury is unusual. Even there, it's not unique.
Stars with planets distributed more widely, like ours, have been systematically filtered out of the results. We have no idea how common they are. It might be that they are rare, and that would make us unusual. In that case, distribution would be interesting and relative sizes would be unremarkable.
If in fact they are common, there is no reason to expect sizes in those cases to be uniform.
A system that condensed from a cloud with less intrinsic rotation seems more likely to have its planets clustered close. Less intrinsic rotation could also result in more uniformity.