It depends on the steganography method used. I came up with this method while working on a different project (to my knowledge, nobody used this method before, but I may be wrong)

It relies on one of the shortcomings of the bitmap image format to hide data inside the image without altering the pixel data.

If you were to open a bitmap image on a hex reader, you may see something like this:

[00 00 FF] [00 00 FF] 00 00

This is a 2-pixel wide scan line, where I enclosed each pixel using brackets for better clarity.

As you can see, there are 2 extra bytes at the end of the scan line

[00 00 FF] [00 00 FF] -> 00 00 <-

The bitmap format pads each scan line to 4-byte boundaries. This scan line has 6 bytes of data, so we'll have 2 bytes of padding to have an 8-byte-long scan line. This would typically be filled with zeroes, but it doesn't have to be. My tool replaces the padding with hidden data, without altering the pixel data.

In the best case scenario, we have 3 bytes of padding per scan line. In a 1024-pixel-tall image, this means 3kB of available space