Small download speeds can also be related to their upload speed settings, and also due to them sharing many files & thus their upload speeds get shared around & perhaps squeezed. Larger files for some odd reason do tend to slow down over the downloading period. This may be due to traffic, or the person taking on more uploaders, or the physics of the communication between host & downloader ... communicating which parts/packets have been downloaded & each message getting slowed... You may or may not be aware but ultrapeers you are connected thru can change during a session & some may be less reliable or be within packet shaping ISP areas or pass thru such over the hops of isp's to get to them.