Simply resize it via GUI or CLI
Example CLI: re-sizing to 1g from 20M.
::> volume size -vserver SVM -volume <svm_root_vol> -new-size 1g
On the side note:
SVM root volume is not going to be an issue even if it is 20MB only, most of the space it consumes is 'namespace' only. You may have to worry about "node's root volume"
Multiple issues can produce a node's root volume running out of space.