MacOS doesn't really need defrag for several reasons. First, the underlying block allocation schemes used in HFS are far better than FAT and NTFS (the filesystems used by Windows).
Second, even with better algorithms, you will get fragmentation *but* MacOS automatically defragments files smaller than 20MB. In doing so, it will, over time, clean up the little files scattered everywhere that cause big files to get fragmented in the first place.
Third, modern disk drivers are far better at dealing with fragmented blocks. "Back in the day", you made a request for a block, in the order it was received, waited for the heads and disk to rotate to the right spot, and then read your data. Now, the disk driver chunks up requests in whatever order it determines is best (and preserves the semantics of what you're doing to the file) and then the disk itself can actually cache and reorder data. While I haven't done a test, I suspect that defrag tools on Windows don't get nearly the performance boost that they used to.
Finally, on an SSD
(which many of the Dell Mini's have), fragmentation at the level of what you're likely to see simply isn't an issue at all. Because the media doesn't rotate, you don't get appreciably more performance by having no fragmented files.
This article (not written by me) goes into detail on actual performance numbers with HFS and fragmentation: Fragmentation in HFS Plus Volumes