Offline quality differences between PC and Mac

We are having a hard time getting consistent, quality offline downloads.  Our experiments has revealed that the final size of the recording can vary greatly - for example, on a PC the final size was about 90 MB, while on a Mac the same meeting finished at 130 MB.  These computers are on the same network, even physically, and have comparable components (i7 Processor, equivalent RAM).  This kind of difference has been consistent and repeatable, regardless of the time of day.  The quality of the Mac download is, unsurprisingly, better.

Any suggestions as to what could be causing this difference?

Thank you!