I think it has to do with body mass. Plants lose body mass underwater, even sword plants do this. I think by losing body mass they are able to take up nutrients faster and go thru photosynthesis faster. Those plants that are not able to adapt and do not lose body mass, such as Anubias, continue to grow very slowly under water. But to go from a needle like leaf to a round leaf is rather amazing.
In the air, plants need to protect themselves from water loss, while still getting CO2. CO2 can only be picked up by wet cells, and wet cells lose a lot of water. The plant can't expose large areas of wet cells to the air. It would lose too much water, and the wet cells would be unprotected. The compromise is stomata, little openings in the leaf through which CO2 diffuses in, and H2O diffuses out. They minimize water loss while still getting a decent amount of CO2. The outer surface of the leaf is covered with a waxy material that reduces water loss. Nutrients are obtained mainly through the roots in emersed plants, and so the roots have a huge surface area to increase uptake from the soil.
Put the plant in water, and now water loss is not a problem, but CO2 uptake is a big problem because CO2 diffuses through water about 100,000 times more slowly than it does through air. CO2 in the water is picked up through the leaf surface, and so many plants increase their surface area to compensate for the low rate of CO2 uptake per square inch. Mineral nutrients can now be obtained from the water, and increased surface are is a benefit, here, too.
Plants that live both emersed and submersed have to undergo an actual metamorphosis to switch from an effective form in one medium to an effective form in the other. Just as a caterpillar has the genetic makeup of a butterfly hidden in its genome, or as the tadpole carries the genetic makeup of a frog, so does the aquatic plant in many cases have the genes for a second form . You can't tell by looking at the terrestrial form that it has or has not the capability of switching to an aquatic form. You can only find out by sticking it in the water.
I found in a nearby ditch with standing water some emersed plants that look like pictures of emersed Limnophilia. It had the properly shaped light purple flowers, but the leaves looked a lot bigger than those in pictures of emersed Limnophilia aromatica. I broke off about the top twelve inches of the plant and stuck it in an aquarium. So far, it hasn't died back very much. I think I will get a 'before' picture. Maybe the 'after' picture will be a dead plant. Maybe it will show a talent for underwater living.