Hello @sibert and welcome to the community.
You can set the size / disk quota of a virtual server to any amount of space that you like. The 1 GB default can be changed to a smaller or larger value, so yes, the virtual server can be smaller than 1 GB.
If your content is 15 MB then you can set the disk quota / space for that virtual server to be 15 MB, or better yet, to 20 MB so that there is space available for the extras such as sundry temporary files, SSL certificates, perhaps a cache and most certainly for log files and their archives. If a virtual server runs out of space then it will generate an error and might not function normally, so it is best to leave a little extra space when creating a virtual server or editing disk quota.
You are aware that Virtualmin permits you to ‘oversell’, so if you have a 1 TB disk, for example, you can create as many virtual servers and assign each a disk quota / space such that the total exceeds 1 TB or even 10 TB, Virtualmin does not prevent you from assigning more disk space than you really have on the server. Of course, the actual space used by all your virtual servers must be less than or equal to the actual space on the disk - i.e. 1 TB in our example. I should also add that it is best not to oversell (or oversell too much) so that your management skills are not called upon when demand exceeds supply.