I always like Chanoma's art, i'm guessing the difference between both of the girls are one dream big and ambitious but hard to achieve (and almost unreachable), while the other dream a smaller and simple but attainable.

I think the author is imitating this:
https://en.wikipedia.org/wiki/Ebbinghaus_illusion

Pretty sure it's the same girl at two ages, as she got older (and bigger) her dreams got smaller.

Also, the English title on Tumblr is "Which looks bigger?"