The smaller notes you see are "optional notes", which I like to put in some of my transcriptions.
These are notes that I think should be included, but for some reason I don't think they should be required, e.g.:

 - there's a really wide span that someone with smaller hands might have difficulty playing
 - a layered rhythm is really tricky because a short note is in just an awful spot
 - I'm transcribing from sources of different versions of a song, and merging notes from each would make it really hard
 - I think the song would sound better with a thick or dissonant chord that wasn't played in the source I'm transcribing
 - a note doesn't sound as good on the piano as it did with the original instrumentation, and I think it would sound better without it.
 - I just felt like making a note optional, or perhaps my cat did it when I wasn't looking  >^,^<

