Why JPEG files get large
Modern phone cameras capture 12-50 megapixels, so a single JPEG can easily exceed 5 MB. JPEG already uses lossy compression, but the default quality from cameras is very high. Lowering it to 75-85 usually halves the file with no visible difference.
Chroma and quality explained
JPEG stores brightness in more detail than colour because eyes are more sensitive to light than hue. When you drop the quality slider, Aimager re-encodes the JPEG with a fresh encoder rather than simply relabelling the file, so the reported size matches the real download.