There are microlights and there are microlights.
Some of them are so flimsy it's a wonder how it hangs together. Yes it may be stressed to +4G but then I am sure a hang glider will withstand +4G.
Others look a lot stronger. A 750kg one is likely to be a lot tougher than a 450kg one.
One could build a tremendously strong all-composite plane within 750kg MTOW. But it would be too expensive for the market.
I've done a great deal of mechanical engineering over the years and would never get inside the traditional English ones, e.g. a Thruster.