Quoting the first of the surprised replies to this I was with an employee at XK a few weeks ago who told me that they could not go above A330s at AJA without going into detail why, but I'll ask next time I see him.

Indeed 747s have often flown to AJA in summer and I think even just a few years ago Corsair did a special service for XK on a few summer weekends !

As for runway length, AJA is 2407m and BIA 2520m, which isn't a huge difference, but the rwy 20 approach in AJA is trickier due to the mountainous approach. Only the B taxiway is cleared for wingspans up to 65m which also constrains usage for large aircraft.

I think the main sticking point has to be the terminal, which is a lot bigger airside at BIA than at AJA and has been recently expanded, as well with a bigger security filter (up to 6 lanes at BIA vs 2 at AJA). AJA is due for a terminal expansion any moment though I haven't heard much about the plans yet, FSC seems to have priority from the Corse-du-Sud CCI as the terminal is way beyond design capacity.