What a BMP file is and why it is so large
BMP is the native bitmap format of Windows. In its common form it stores every pixel as three uncompressed bytes (blue, green, red) after a short header, with no compression at all. That makes the size easy to predict: width times height times three. A 1920x1080 screenshot is about 6.2 MB, a 2480x3508 scan of an A4 page at 300 dpi is about 26 MB, and a 12 megapixel photo is around 36 MB. The files come from older versions of Paint, scanner software, industrial and microscope cameras, game modding tools, embedded systems and various Windows utilities that still default to BMP because it is trivial to write.
The upside of that simplicity is that nothing was ever lost. The downside is that email, chat and most upload forms choke on the size, which is where JPG comes in.
What changes when it becomes a JPG
JPG is lossy. The encoder used here is mozjpeg compiled to WebAssembly, running in a Web Worker on your machine. It converts the pixels to a luminance and two colour channels, subsamples the colour, splits the image into 8x8 blocks and discards the high-frequency detail that the quality setting says you can live without. For photographs and scans this works extremely well because that detail is largely noise. For screenshots of text, spreadsheets and diagrams it works badly: sharp edges produce faint ringing and blocks that are visible at normal zoom. For that kind of BMP, use BMP to PNG instead and keep the pixels exact.
BMP carries almost no metadata, so nothing meaningful is lost on that front. Colour is treated as sRGB. The rare 32-bit BMP with an alpha channel loses its transparency, since JPG has none; transparent regions in canvas memory carry no colour and typically come out dark rather than white.
Choosing a quality setting
The slider runs from 20 to 95 and starts at 80. For a photo or a colour scan, 80 to 85 is the range where the result is visually indistinguishable from the BMP on a normal screen. Go to 90 or above if the file will be edited again later, since every re-save of a JPG loses a little more. Values below 60 are only worth it for thumbnails.
A worked example: that 26 MB A4 scan of a printed document typically becomes 1.5 to 3 MB at quality 85, and 1 to 2 MB at quality 75. A 6.2 MB 1920x1080 game screenshot, which has lots of smooth gradients, usually lands between 300 and 600 KB at quality 80. A grainy 36 MB camera capture may still be 4 to 6 MB at quality 85 because noise is expensive to encode. Treat these as typical ranges rather than guarantees.
Limits of doing this in the browser
The whole image has to sit in memory as RGBA, which is four bytes per pixel, plus the source buffer and the output. A 36 MB BMP needs well over 100 MB of working memory in the worker, which a desktop browser handles easily and a low-end phone may not. If a very large file fails, try it on a computer or split the job. File size limits explains what is realistic on each kind of device.
Browser decoders handle standard 24-bit and 32-bit BMPs reliably. Less common variants such as RLE-compressed 8-bit files or unusual bitfield layouts may fail to decode; opening the file in Paint and saving it again as a 24-bit bitmap fixes that in nearly every case.
When not to convert
Do not convert a BMP that you still intend to edit heavily, a master scan that you may need to re-crop, or a pixel-exact reference for software testing. JPG is a delivery format, not a working format. For those cases keep the BMP, or move it to lossless PNG. And if the goal is simply a smaller photo for the web with control over dimensions as well as quality, the compressor does both in one step.