Reels have their own encoding pipeline
Instagram treats Reels differently from feed video. Reels are optimised for fast repeated playback in an endless scroll, which means a shorter GOP, aggressive compression on static scenes and a target resolution of 1080x1920. The upshot is smaller files than a comparable feed post, sometimes visibly so on footage with a lot of fine detail.
Grabnode pulls the exact rendition Instagram serves the web player, so nothing is added or re-compressed on our side.