Enough room for a readable document
50 KB is tight for a photo but comfortable for a cropped document scan, because a page is mostly white and white costs almost nothing to store.
In our tests a busy, detailed 3000 × 2000 photo came out at about 875 × 585 pixels at 50 KB, while an image with a plain background kept about 1,070 × 1,340. A scanned page behaves like the second case: the writing is a small share of the picture and the paper is almost free. That is why a certificate or an ID card scan can stay readable under 50 KB, provided the scan is cropped to the document and not surrounded by the desk it was lying on.