It's to do with frameshift drives as well as the mass of a nearby object.
PLUS, the ship's computer is programmed to ensure it is a certain distance from a station before frameshift drive charging is allowed.
Imagine the space around a station is a big rubber sheet.
The station is a big ball on that rubber sheet
Ships are really SMALL balls, HOWEVER the presence of a frameshift drive capable of moving the ship means that they're really HEAVY small balls (not mass heavy, but frameshift energy heavy). As the interference from the drives is a big nono. Fire two up that can interfere with each other and you've got a world of ripped apart reality, witch space interference, it will all go a bit Event Horizon really quickly and people will turn inside out. Bad juju.
Large stations contain lots of frameshift capable ships. Luckily though, the effect isn't additive when it comes to the exclusion range for jumping.
So - it's not directly related to the mass of the ship that's preventing you jumping, more the effect of the frameshift core that's required to MOVE that mass.
Similar reason why passing large stellar objects causes you to slow down - your frameshift drive REALLY doesn't like being interfered with so its output is mitigated somewhat when passing a large gravity well.
Anyway - frameshift drives, jumps and similar are bad news if a drive fires up in proximity to another frameshift core. Something to do with sympathetic behaviour from the core of an uncharged one, and the core of a charging one. Same reason why Hutton Mugs get warm whilst in supercruise. They're made from the scrap frameshift drive plates from ships that didn't make it to Hutton without running out of fuel. Fire up your frameshift drive with one on board, and the mug appreciably warms up.