10 million people could download a torrent file, but if the uploader has a slow internet connection, then it won't make it on the list until other people start getting it and spreading it around.

As I understand it, it's download traffic during a certain period, as recorded by the tracker.

I think you're right about this. But this makes it harder for smaller torrents to get to the homepage.
Take two torrents that both get 100 downloads, one is 500mb, the other is 5GB:

500mb X 100 downloads = 50GB download traffic
5GB X 100 downloads = 500GB download traffic

Same number of downloads, yet one is 10 times as 'popular'.

I often see torrents that are 'snatched' a lot of times but because the file size is less than 200mb they don't make it to the most popular list.

If we look at 2 torrents; 1 large and 1 small.  They could both be on the list because they have the same amount of download traffic.

Larger torrents are popular because they are either better quality and/or they are packs.

I downloaded a video from about 15 years ago that I love, but it's so small in my player that I need a telescope to see it on my laptop.    If there was a larger version it would be great.

Some people love single videos and won't download packs.  Others have ratio issues so they go for the smaller file sizes.

I suspect it takes total traffic in to accounts because they seem to be weighted towards torrents that are larger in size getting to the list.

Another torrent could have 10 times more downloads but be inactive, so it won't be on the list.