If you have shell access you can use command line tools like 'top' or 'htop' to find the processes using much RAM. Misskey itself is using around 0.9 GB on my instance. The database usually needs a lot of memory for buffers and caches. When you're using elasticsearch with Misskey you have another candidate for eating up RAM.
But in general, using a lot of RAM isn't a bad thing. The memory is there to be used. Or in other words: in Linux unused memory is wasted memory. The only bad thing is overused memory because it will lead to heavy swapping - which is bad for the performance.