MM - depends which brands you are after..
For example - DS is 12.5kg, Miss M is ... gee I dunno... maybe 5.5?

They both fit into the medium Itti Bitti's..
DS fits into medium Widdlewuns - they are far too big for Miss M - the rise is quite high on them.
DS is in the Large Swaddlebees - mediums don't fit Miss M yet.
(Racking my brain trying to think of other sized ones I have..)