为什么Mem Usage值会随着时间的推移而增加？

I have a process running 24 hours on my system comtinuously for 45 days. In the morning the number of threads are 20 and in the afternoon the number of threads are 100 and in the evening the number of threads again comes back to 20. But the Mem Usage and VM size values does not come back to the values when the number of threads were just 20... The values goes on increasing everyday untill i restart my system? what is the underlying matters in this ? please explain me .... I have a process running 24 hours on my system