That's because you're trying to return the entire value of the binary variable, but the entire point of binary variables is to hold large amounts of data that mIRC commands cannot usually interpret.
Instead, you should return the name of the binary variable, and then in the command where you actually NEED to use the data (ie. writing it to a file), then you can either parse the data in chunks, or something else depending on what you do with it.
It's hard to help you without seeing where you actually use the data.
