Right after posting this question I managed to find a bunch of apps which do something like this. I will post what they are and any advantages/disadvantages I see from them (also, please note that my mini reviews are only accurate as of my writing this; features may have been added or annoyances fixed after I wrote all this).
Photo Cleaner, $1.99
The app I was originally looking for. It has multiple features such as the ability to detect duplicate and similar photos, and it allows you to browse your screenshots (while seeing their file sizes). Most importantly, it shows you your photos/videos in a grid format with their sizes on the icons, allowing you to decide which ones to keep/remove.
However, it doesn't seem to go much further than that. Even though you can see screenshots' file sizes which is helpful, it doesn't allow you to sort the screenshots by file size, which would have been greatly helpful. You also can't inspect the photos from the "similar photos" feature and see which one you'd like to keep over the other(s). Surprisingly it doesn't even allow you to see the date/time the photos were taken which would allow you to look them up in the photos app.
The way I worked around this was by deleting a photo, going to the trash where it is kept (third party apps are not allowed to permanently delete photos – they will always be sent to the trash), and looking up the date from there. This is however inefficient and provides a bad user experience.
Something else I noticed is that videos don't seem to play when trying to view them from within this app, most likely a bug because they seem to play file with the other apps I mentioned.
Video Compressor, Free w/ IAP
This is actually a video compressor app, but shows you your biggest photos/videos as well as their file sizes. However, it doesn't allow you to delete or mark the photos/videos in any way, or even allow you to look them up in the photos app (no date is shows). So for this purpose it's not very useful.
iCompress, Free w/ IAP
Although also a compressor app, works much like Photo Cleaner without the extra features. Has the added benefits of being able to play videos properly and being able to see extra information about the videos, like creation/modification date, duration, resolution, frame rate and bitrate. Probably one of the best apps for the job, in my opinion, especially since it's free. However, it doesn't show photos – only videos.
PhoTop, Free w/ IAP
The app I mentioned in the question. It's quite good for seeing your biggest photos. In this way, it can be used in tandem with iCompress which shows only videos (alternatively, you could use Photo Cleaner to deal with photos if you're set on using iCompress).
In any case, one would preferably try all of them out to see which ones work best for them. The reviews I wrote are of course subject to my opinion, and although that might help someone make a decision, these apps are likely made for slightly different people/workflows. Use whatever works for you!