Theres no mystery to it.
The relatively short hop to Toronto from GLA ( sometimes done in 6 hours) would take off at nowhere near MTOW. The fuel which comprises as significant proportion of the weight at takeoff would be relatively small on this particular route.
Again this particular route carried relatively little cargo ( aimed mainly at leisure travellers in the summer)
Very rarely did the AC 742 from GLA cross the Atlantic full either.
747's can easily operate from GLA but their performance is strictly limited hence the reason 747's at GLA are much less common than other types.
Im sure even an A380 in theory could operate from GLA if it was empty ! I can see Virgin jumping at the profits they would get from that.