When I do "top", I only see that mysqld is using 70% of Memory. Nothing else.
Why is it being used up?
Linux takes advantage of all the memory available to it.
If there is memory that is not being specifically used by applications, the OS will use it for cacheing to speed up other processes.
No use letting useful resources sit there unused :)
Mysql (and any other SGBD) are trying to load the most data possible in memory because it is some much quicker to get data from RAM than an hard drive.
This is a normal behavior on Linux (probably most unix, and maybe windows ?) systems behave like this. The more you have in memory, quicker the software runs.