Well... that's part of it.
We know how big a tree is. We know how big a house is. Sure, they're all different sizes, but the *average* is very well set in our minds, from a rather young age. So, if we see a picture with a tree, a car, and a house on a floating island, with a waterfall over the side from a small stream , and those three things fill the entire island, we know that the island is REALLY small... about the size of your backyard... definitely no bigger than a city block.
If the car - drawn exactly as it is - was appropriately sized beside the house, and the TREE was smaller than the car, we wouldn't think "tree", we'd think "bush", because, regardless of whether it's a map, or a painting, or a photograph, our minds know how big those things should be, in comparison to the tiny bridge, and the big house.
Same thing applies, or should, conceivably, to mountains and islands, continents, and coastlines. So people make maps that seem size-proportionate, and we view those maps and it adds to the depth perception we've been building since birth -- or rather... since we developed a vague understanding of the world outside what we've seen.
My 4 year old draws a picture. It's him, his mom, a house, and a mountain. There's no correlation in sizes. Usually, he's the most important thing, so he's biggest, then mommy, then the house. The mountain is either a big scribble off the page, or a tiny little thing off to the side. He knows "mountains are big" but he doesn't have a perception of that meaning yet.
EVERYthing in his world is big.
By around 5-6 years old, a kid will (typically!) draw himself bigger than mommy, a house bigger than both of them, and a mountain the size of a house.
If they live NEAR mountains, and see them every day, mountains tend to be semi-size appropriate
Another year and things go in order of size but they still aren't proportional. And then we start learning - really learning - geography, and once we understand houses are big but mountains are HUGE, the kids will draw towering mountains, medium-small houses and tiny people.
You can see how this affects depth perception by watching them fall. Little kids trip over *nothing*. They're just starting to learn the difference between a "big step" or a "little step" or a "giant" or "baby step".
As we get older, and our perception evolves, we balance more easily.
So... where was I going with this? Oh yeaah..
So we see a mountain on continent A). And it's about an inch and a half across. and we see an island near continent B) and it's about an eighth of an inch (didn't pull examples from your map, just explaining an example) and we think, ....well we don't really think... our mind just processes it, like any other size/depth perception... like:
Okay, the island is 1/8 the size of the mountain, must be a really big mountain or a really small island"
But if ALL the islands are similar sizes, and ALL the mountains are similar to the first we can't rationalize that. So we think "What a huge land. What gargantuan mountains!" "what a tiny coast!"
And your brain either just sends out a signal: "Something is off". or, depending on your visual experience, it says, the coasts are right, compared to the islands... so it must be the mountain!!! (or from the earlier example: "the car and the house are okay, it must be the tree!")
So even if the mountains are perfect, and everything ELSE is off, we see the mountains as flawed.
I did TELL you all I ramble... :/