mongodb is using most of disk space for journal and not cleaning itself
MongoDB (for MMAP storage engine) will allocate 3 journal files by default at 1GiB each.  That's where your journal related space usage is coming from, but it will not grow unless you have a very high insert rate.

You can start with the smallfiles option and reduce the size to 3 x 128MiB if you wish, but be aware that your data files will also be reduced (to 512MiB each) so there will be many more of them and they will need to be allocated more often when adding data.

As for whether to increase your storage, that depends on how much data you intend to add to the database - it will need to allocate new data files to store any data you insert, so it really is dependent on your planned usage as to whether 10GiB is enough or not.