I think most jams are related to handling and storage issues. Like lump charcoal, throwing bags around or moving them a lot it can break up the pellets, causing a lot of small pieces and dust. High humidity can also be a problem, especially over longer periods of time. As for large pellets causing problems, I agree with
@LMichaels.
Over the years, I've probably used more Pit Boss pellets than anything else but have also used Bear Mountain when I find them locally, as well as many other brands and haven't had any of them cause a jam. The only jams I've had have been after leaving the grill uncovered during severe thunderstorms. The fact that I don't sift out dust or small pieces may have been a contributing factor. I do live in the high desert, so humidity generally isn't a problem.
I can't speak specifically about Webers, as my only pellet grill is a 12 year old Traeger. Most pellet grills use the same, or similar auger configuration as Traeger, so those who use different pellet feeds may well have problem more related to the grill's ability to feed pellets with certain characteristics.
Many manufacturers recommend using their own brand of pellets. Some even say that using other brands will void their warranty, even if they don't make their own. I understand the blame game and am pretty sure it was started by Traeger and proliferated by many other manufacturers, primarily to sell more of their brand of pellets. I've never heard of any manufacturer voiding a warranty because other pellets were used.