I was reading this article that mentions a blackhole as having
a gravitational sphere of 4,000 light-years.
I'd not heard of the term (gravitational sphere) before so looked it up, and it looks like a simple definition is the same as sphere of influence and according to Wikipedia is
A sphere of influence (SOI) in astrodynamics and astronomy is the spherical region (actually is an oblate sphere) around a celestial body where the primary gravitational influence on an orbiting object is that body.
From the above, I take it to mean that an object could be tiny, but providing there are no objects anywhere near with another object with a bigger gravitational pull near them, then its sphere could be larger than a more massive object. i.e. I can't directly look at a gravitational sphere of 4000 light-years and think "that must be a huge object".
Is my understanding correct?