We add a little extra to the size in some spots since emails when they are encoded tend to use a bit more size then what you'd expect.
Base64 encoding can increase the data size by roughly 20-30%, we perform this formula to calculate the extra size, (bytesIn * 4) / 3

There does appear to be some mis-alignment in this behavior in this area. I'll add a task to correct it, haven't looked at the code yet but there is a chance the text in this response is just wrong, but the actual logic is still correct.