The approximate size for a Static Mesh that is shown in the tooltip from hovering over a Static Mesh in the Content Browser and the approximate size shown in the Static Mesh Editor can be inconsistent. This is due to the former rounding to the nearest int while the latter casts directly to an int32.
The following are the two relevant blocks of code.
Asset registry tag portion:
Static Mesh Editor portion:
This code was also present in 4.14
N/A - Code issue
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-44645 in the post.