In the worst case (with alternating black and white pixels) the run length encoding method will result in a file that's larger than the original!
As noted above, *every* lossless compression method that makes at least one file smaller must also have some files that it makes larger – it's not
mathematically possible to have a method that always makes files smaller unless the method is lossy.
As a trivial example, suppose someone claims to have a compression method that will convert any 3-bit file into a 2-bit file.
How many different 3-bit files are there? (There are 8.)
How many different 2-bit files are there? (There are 4.)
Can you see the problem?
We've got 8 possible files that we might want to compress, but only 4 ways to represent them.
So some of them will have identical representations, and can't be decoded exactly.

Over the years there have been several frauds based on claims of a lossless compression method that will compress every file that it is given.
This can only be true if the method is lossy (loses information); all lossless methods must expand some files.
It would be nice if all files could be compressed without loss; you could compress a huge file, then apply compression to the compressed file, and make it smaller again, repeating this until it was only one byte – or one bit!
Unfortunately, this isn't possible.