Growing Purps... about four weeks in... they look small?

I went through my first cycle... did 2000 watts of Sour Diesel and yielded a bit less than 2.5 pounds. I am reviewing my old pictures of the diesel at four weeks and they are a ton bigger than my current purps (will post pictures manana)

The Diesel took 67 days to finish, while the Purps apparently takes around 56 days... I am curious if any of you have grown purps before. I have seen em blooming in late stages, wonderful smell...good looking plants but mine simply haven't taken off yet.

I used MOAB last time, and did not this time (I will be using shooting powder for the last three weeks of flower) I am curious if quicker strains start quicker than slower strains or really it just depends on the strain?

Any information on Purps is appreciated.