[saurik]

One thing that is important to point out that is that JavaScript (and thereby JSON) doed not separate integers from floating-point numbers; so if you have the double 3.0 it will be represented in JSON as "3", not "3.0". If MessagePack does not have a mechanism for dropping down to int32 for integral doubles, then you will actually see a 9:1 difference for this specific case (although if you are often storing integers in your doubles maybe you should be using int32 anyway, in which case 3.1 may have been a better general example than 3.0 ;P).

[stock_toaster]

I think that depends on the library you are using. 3.0 (not truncated to simply 3) is certainly a valid json number. However, point taken that it is a meaningful distinction for javascript.

[saurik]

Oh, it is certainly valid; it is just that for purposes of comparing relative space you should be looking at the shortest representation. "3.0000000" is also valid, and is the same 9 bytes as MessagePack, but there is no reason you'd use that to encode this specific number ;P.