Checking Folder sizes

This is because the Get Info displays both the data size (logical) and size on disk (physical) of a given Finder object. The logical size is the actual size in bytes. The physical size can vary quite a lot given a variety of conditions. Example: a SimpleText file of zero bytes takes up 65k of disk space. This is due to how the disk stores files (i.e. Standard or Extended format), how ‘well’ the file fits into the formatted disk blocks, whether it has a resource fork, etc.
Anyway…when you say,
: …(when a) folder contains 600 MB of data.
I assume you are setting up a backup to CD-ROM or something similar, so the size of the file ‘on disk’ is what you need, not the file size in bytes. So, you have to script the information window of the Finder (see Finder Dictionary) to get that size.
your script line,

: set theSize to size of (choose folder)

should go,

set theSize to (physical size of information window of (choose folder))

Also, I found the method that Sal Soghoian put in his ‘SIZE OF FINDER SELECTION’ script to run faster than your dotheMath sub routine. (with slight modification) Evidently the offset command slows it down (I’m guessing there).
So, for dotheMath sub routine, try,

on doTheMath(theSize)
set the first_part to (the theSize div 1048576)
set the second_part to (the theSize mod 1048576) div 100000
set the theAnswer to (the first_part & "." & the second_part as string)
end doTheMath

Hope that helps.

When I said:

I was incorrect. I was just quoting (and poorly at that) something I once read in Mac Secrets (disk chapter). On my machine an ‘empty’ (not zero bytes) SimpleText document takes up 4k of disk for 332 bytes. I have an Extended formatted drive. It would be more for a Standard (and even greater, proportionaly to size of drive).