The S50 produces two types of files, compressed JPG which are typically the size you mention (and even smaller *_thn.jpg thumbnails, which are useless) and uncompressed FIT files, which are always the same size, 12,159KB - this makes sense, 2MP sensor, 16 bits/color component, three color channels, that's 12MB. ZWO also calls these RAW files. On top of not being compressed and higher pixel size than JPGs, they have a header with very useful information about the captured image. You need either a FIT viewer utility (ZWO has one) or a post processing application like Siril and Graxpert (free) or PixInsight or APP (not so free).
