Yeah, well that's because they usually are
But yeah, look at Cagney or Robinson. Both were about the same height as Ladd was and both were constantly paired with average height women who weren't much shorter than they were and it worked fine. Then again, I've heard that Ladd was more like 5'4 than 5'6 but it seems like no one is certain. Robinson was about 5'5 and Cagney 5'7.

Not exactly, but in films women often appear shorter than their male partners.
"Two more swords and I'll be Queen of the Monkey People." Roseanne

Regarding the heights of Ladd and Lake: Ladd was in fact 5' 6'' whereas Lake was just under 5'. So they weren't matching heights.

There it is! As Nordic as you can get!
"Two more swords and I'll be Queen of the Monkey People." Roseanne

Well Veronica Lake's father was Danish, and Alan Ladd's distant ancestors were Danes or Swedes who settled in Normandy before helping to take over and ruin England maybe that has something to do with it?!
"Nothings gonna change my world!"

Yes, that's true about their heights. The pairing worked on many levels. Sadly, I've read that both were alcoholics.
"Two more swords and I'll be Queen of the Monkey People." Roseanne

And for the studio there was an added bonus: their heights matched.
In the image machine of the time, Ladd couldn't be paired with a woman as tall as Maureen O'Hara or Lauren Bacall or Ingrid Bergman.
And Lake would have looked like a little kid next to an actor with the height of John Wayne or Jimmy Stewart or Vincent Price. (Two-shots of Lake and one of those guys, with both of them standing up, wouldn't have worked.)
Paired with each other, they looked right on screen. Most of the time, you won't even notice how short they both are.