DFC,
Perhaps the UK microlights are stressed for higher "G", but that does not make them "stronger" in all senses. If you were to collide with unwelcoming terrain, the more substantial fuselage structure and skin of an airplane would offer much greater occupant protection. To me that is an equally important "strong". You can nearly always control how many "G's" you're pulling, you may not always be able to control what you're going to hit!
Cheers, Pilot DAR