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microwave ovens haven't used a proper seal in years. they discovered that if you make the chamber an odd multiple of a quarter wavelength it acts like a shorting stub and you see no RF at the plane of the door. in theory.

we don't live in theory

The door still has to seal whether you can see one  or not.  If the "seal" is leaking, RF can escape.

Newer door-interlocks should help with this problem, but they only help on the opening side of the door.  Bent or sprung hinges on the other side of the door can still leave gaps that leak.

Cavity size doesn't mean much if the overall shielding of the oven is inadequate. 

There are still plenty of old microwave ovens about.

If in doubt, test for it.