Adding to what @DRM said, the size increases because the source file uses IPB compression. That means one frame is fully described, and then the next 120 or so frames are described as differences from the base rather than being independently described. Since differences are usually minor, this saves a lot of space. So, to view frame 40, the base frame would have to be read and then the 39 differences after it applied to reconstruct frame 40. That’s why editing IPB video is slow. The conversion process makes huge files because every frame is independently described which undoes all the space savings of IPB video, but makes it much faster to seek and edit because no reconstruction is necessary. So, the size explosion is very normal and expected.
This also explains why 60fps video will be twice the size of 30fps video after conversion.
IPB is also called Long GOP video if you look it up elsewhere.