A pretty good short level.
For the most part the level is calm and easy, but there's several minor cases of Big Mario Discrimination that come up. (As well as a few other things).
In some of the rain's frames of animation, some of the rain becomes black. I don't think it was intentional, but if it was it looks a bit weird.
(unless it's raining oil or something)
This spike top gets to this block and gets stuck. It looks a bit silly. Also another case of issue #1.
Same problem occurs in both of these spots. For whatever reason, Big Mario is unable to get under the block above him (without ducking) even though it looks like he should. Make sure Mario can get through these areas with ease, big or small.
This log moves too fast. After multiple attempts I have NEVER successfully gotten the mushroom or leaf in the item box. On top of that getting to the other side of this chasm is a chore with big Mario or Yoshi as the timing on the jump has to be pretty precise. Granted it is easy to pull off if you have the leaf, but if you don't it's pretty difficult, especially after going through the calmer nature of the 1st half.
Might just be me, but I think this jump is a bit more difficult to perform as Big Mario, might want to move those up a bit (donut blocks).
And finally, I think you should delay the goomba generator by just a tiny amount. It's a bit annoying focusing on the bat swooping down to kill you and jumping over an endless goomba army.
Anywho, good work so far.
Any questions or comments? Don't hesitate to ask.