The manual file sizes have a restriction. Below 12MB, if you download the information too quickly, TMN will send you to larger size.
Judging from the speed of your connection 1 MB should be sufficient... but you may be getting bursts that cause the program to want to throw you a larger size. See this result...
https://testmy.net/db/pPVrZcbX8
I'm surprised that one didn't forward, you must have been right on the edge.
If you were to run 2.5 or 3MB tests I think you'd avoid any forwarding, I think that should stretch it out enough without going overboard. Manually select 3 MB from the download test or use this URL http://uk.testmy.net/dl-3MB&nfw=1 (the "&nfw=1" is the key to tell it not to forward... but it still may if the size is 12 MB or lower.)
Is 3 MB still more than you care to test with? Let me know how that works.
Happy Testing! ... and Happy New Year!
- Damon