Is there any way to cut down the size of an app without upgrading to pro? I am working on a very simple game in unity(so far it is just two scenes, with a couple of pngs less than 500kb in total and a moving sphere), and when I exported it to Xcode and ran it on my device, I found that it was already 18mb. Which, after apple adds its useless drm, means that it will be over 20mb. So, is there any way to reduce my apps size? Also, if I don’t use some assets in the project window does Unity automatically not include it into the final app?
No there isn’t. Unity on its own is without stripping about 16mb + whatever you add in as 3rd party assembly.
And unity will not include files in the build that are not used (unless they are in resources folder) so yeah if you cut scenes etc the size will shrink
By resource folder do you mean the asset folder? Because I don’t use the Standard Assets, so if delete them out of my project then my app size will decrease?
No by resource folder I mean the Resources folder you would add within the assets folder if you want to use assets through Resources.Load - as unity can’t decide which of these are used or not (due to code only dependency) it would include the whole folder.
Having them in the assets folder but not the resources is no problem, unity will only pick up assets there that are used 
Ah ok, thank you! I didn’t even know that existed!