Drop a BMP above, get a JPG back, at a small fraction of the size.
Bitmaps almost always store every pixel in full, with no compression. The arithmetic is fixed: a 1920 by 1080 bitmap takes about 6MB whatever it shows, and a plain white one takes exactly as much as a photograph. That is why they fill drives and bounce off email attachment limits.
JPEG discards detail your eye is least likely to miss, and the same picture arrives in a few hundred kilobytes rather than six megabytes. The trade is real but usually invisible at normal viewing size.
Where BMPs come from: Microsoft Paint saved them by default for years, older scanners and fax software produced them, and a lot of industrial, laboratory and till-receipt equipment still writes nothing else.
How much you save depends on what the picture contains. A detailed photograph usually lands ten to thirty times smaller. A screenshot or diagram often shrinks even further, because flat colour costs JPEG almost nothing to store, and the price it pays there is smudged edges.
If the pixels matter, use BMP to PNG instead. PNG compresses without throwing anything away, and screenshots, diagrams, scanned text and anything with sharp edges stay crisp. JPEG softens exactly those edges.
Batches convert at once and arrive as a ZIP.