One can have no smaller or greater mastery than mastery of oneself.
I believe this means that mastery of oneself is crucial and everyone should have it, therefore no mastery of anything "smaller" than that. However when you have that smallest necessary mastery you can go forth and start mastering other subject and therefore it is also the greatest.
In other words if you do not have mastery of yourself then you cannot even start knowing yourself, and maybe I'm going to far, but you wouldn't be a person at all. So it is the minimal mastery. But when you have it, it enables you to further your research also making it the greatest pin and so it is also the greatest mastery.
Most dictionaries list two distinct meanings of mastery:
- the authority of a master
- skill or knowledge that makes one master of a subject
One intepretation of da Vinci's quote is that mastery of one's self according to the first definition is trivially easy for most people, whereas mastery according to definition two could take a lifetime.