On mobile it should load the mobile-size version, which is much smaller than what you see on the desktop.
Would you consider making a pull request with an alternate theme (there are two already included)? You could adjust the styling as you see fit, and so forth. Best of all, were you to actually use this, you could modify the compression settings and even the scaling in a way that fits your needs, even if you don't end up making a PR on the original.
Looking at this, there are lots of ways where it feels like there could be things added to this to make it better -- a photo reorganizer, or something that would let you auto-tag / order photos from within the interface. Those kinds of things would be really neat, but are likely outside the scope of the (amazing) tool that Jack was kind enough to give us. If one were to go far enough down those rabbit-holes, pretty soon one would have rewritten it as a CMS. ;)