I think, this is more an OS issue (or even an OS resource use reporting issue) than an issue with your program. Of course, it only uses 5 MB of memory: 1 MB for itself (libs, stack etc.) and 4 MB for the buffer. Whenever you do a fread(), the OS seems to "bind" part of the file to your process, and seems to release it not at the same speed. As memory use on your machine is low, this is not a problem: The OS just keeps the already read data "hanging around" longer than necessary, probably assuming, that your application might read it again, soon, and then it doesn't have to do that binding again.
If memory pressure was higher, than the OS is very likely to unbind the memory faster, so that jump on your memory usage history would be smaller.