That's not entirely true... you most probably had the wrong lights - length or power...
Here is a better explanation from a guy that know what he is talking about - I know is from the car industry, but headlight design has the same principle for motorcycles as well.
Is a long video, but worth to watch it. Listen to his explanation on the types of bulbs and headlights housings.
https://www.youtube.com/watch?v=DeaDrH0gXZk