File size is based on encoder, chosen 'compression' ration, frame rate and resolution (and a bit more).
My 1080p30 recording using middle of road quality (compression) settings are around 10GB/hr. If you want better video image quality, either file size goes up, or you use more computational cycle to compress (ex using H.265 vs H.264).... no free lunch
So if file changed, that means your recording settings changed. And I don't mean this in a snotty way, but most likely OBS did exactly what you asked it to do. Maybe you didn't realize the implication of one of your settings changes?