constant memory usage

Utilization depends on what you're trying to implement and if it needs it. It also depends on the resources of specific FPGA you have. If you're operation is complex enough such that a DSP can be inferred, then it will use a DSP. Otherwise, it will just implemented it using logic blocks. For example, if you have a = b + 1. A simple incrementer like this would not need a DSP and would probably be more efficient if it was implemented using logic. Essentially, all this mapping is done by the tools.