I am surprised you said that, because I thought the Weber pellets were unusually long - although some say more slender.
I can only say that my Rec Tec works fine with Weber pellets as well as Lumberjack, CookinPellets.com and Rec Tec's house blend. I have never tried Traeger pellets but have never heard much to really recommend them. Having not used them at all, I am not trying to be critical. Some others are rather critical of them. I am open to other views.
I have been surprised how many people have said good things about Pit Boss pellets. Their Competition Blend gets some favorable comments and is a real bargain at Lowes.
I personally LOVE Weber GrillMaster Blend. My wife does not like smoked foods and even she said the ribs I brought in cooked with Weber GM "smell really good!" They are rather pricey, but to me they smell the best of any pellets I have tried. Close comparison to CookinPellets.com "Perfect Mix" which I also really like.
Lumberjack is also a great company. I have some of their blends I haven't even tried yet. One big plus for them is that they offer a number of 100% pure wood types. So, if you want JUST applewood or 100% cherry, you can have that. (All of Weber's pellets are blends - high quality it appears but not single wood type.) Lumberjack's 100% offerings are also good if you want to make your own blends starting with individual wood types.