For anybody who knows how hard drives work.

I just got a 300gb drive to replace a couple of older 80gb drives.
I'm transferring all the files over to the new drive.
Why is it, that the sizes of the files and folders are different when you compare them.

For instance:

On my old drive I've got a synth folder with...
"139.2MB (145,754,472 bytes) for 131 items"

On the new drive it reads as ...
"139.7MB (145,754,472 bytes) for 131 items"

Why is there .5 more megabytes taken up on the new drive when it's exactly the same size and amount of files that I copied over?