Not really anything to do with memory capabilities, it's more to do with number crunching.
The Bullet physics engine can handle any collision shape but the simpler the shape the better the performance, the simplest shape is a box, calculating collisions between boxes is mathematically pretty simple which is why you so often see demos with thousands of boxes to show off a BE implementation.
Spheres are also mathematically pretty simple, cylinders are slightly more complex as are cones.
Arbitrary mesh shapes are very complex and computationally very expensive so where possible the best approach is to break down a complex model into simpler shapes, GG can do this but IIRC it isn't implemented for dynamic entities.
Convex hulls are a compromise whereby a closest fit approach is used to reduce a complex mesh into a simpler less computationally expensive shape, in a lot of cases the resulting shape will match the visible shape well enough for most purposes.
For dynamic entities GG currently only allows box, sphere and cylinder collision shapes. If I read the OP correctly Preben has added another option, i.e. convex hulls.
Cone shape could also be added relatively simply and with a bit more work I reckon that multiple physics shapes could be added for dynamic entities. But arbitrary mesh collision for dynamic entities are never going to be a good idea!
When I first started playing around with GG there was a way of making a dynamic entity have mesh collision and in my first stab at the moonlander demo I did used the lunar lander model with mesh collision (and nothing else on the level!), I got single figure FPS when moving it around dynamically!
The simplest answer is to choose dynamic entities for your game very carefully to make sure they fit one of the simpler collision shapes.
Been there, done that, got all the T-Shirts!