glCopyPixels Memory problem

Sorry my english is poor,
I have a question to ask.
The program that uses glCopyPixels, runs under different computers has different effects. Running under the computer A makes the consume of memory increasing sharply, while running under computer B is normal. No matter computer A or computer B,running glReadPixels, glDrawPixels and then free(pixels),the consume of memory is the same.
Who can tell me the reasons.