Unfortunately there’s no single answer, it is highly dependent on your assets, document goals, and the target browser/device on which they will be run.
The first two scenes that use native animations in Hype look like they are pretty well structured and the assets are cropped and sized minimally. I think it would probably run well on any device. That said, your document does use flexible layout, yet the images are sized for the iphone layout, thus they will be a bit blurry when scaled up. So it is up to you if you want to potentially sacrifice some load time and possible performance by boosting the asset sizes for if this were to ever show up on an iPad or much larger Desktop browser.
One important aspect to remember is that just because a file size of a PNG is small, that does not mean it will not incur large render times or affect performance. I’ve seen PNGs that are only 50 KB but occupy 10,000 x 10,000 pixels. The browser eventually has to convert this to a bitmap based representation. This would be 381 MB in memory! Just moving that around and then scaling/drawing to the screen will take a lot. If you can, avoid images that are larger than 2,048x2,048. Ideally as small as possible.
Generally speaking I prefer working as “natively” in Hype as possible - that is avoiding pre-rendering to video. This gives more control for preloading and animation. But sometimes depending on the content a video is the way to go.