Norwegian HNoMS Olav Tryggvason too only 280 mines. And she was purposes build minelayer, with heavy armament for it's type...
And she was described as one of the big ones!
Olav Tryggvason was a bit smaller. It was design to fit 2 twin torpedo tube mount, that may have spend space unusable for mine racks.
It's described to use a covered mine deck, this could be less roomy that using a wide open deck.
I think minelayer have very variable design.
Cruiser-minelayer like british Adventure or french Pluton, even being far bigger than the ship we are talking about, carried only respectively 280 and 290 mines (should have roomier machinery to great speed).
on the other and the US Terror, bigger than swedish norwegian and polish ship but smaller than the british and french, carried up to 900 mines.