What do the "smaller width" notes mean in Rockband?

[Everclearboy]

the ones that sometimes appear that are noticeably shorter in width? thanks!

[beatlesfan99]

If you are talking about guitar or bass they are Hammer ons or pull offs.

[ricecake]

Those are hammer-ons/pull-offs. Assuming that you hit the note immediately before it, you can hit the smaller note without strumming. It takes some practice to get the timing right, but once you get the hang of them, it makes playing fast sections much easier.