Ah, I see why you struggle to grasp this now.
Readability =/= detail. In fact, often they are polar opposites. Detail is to do with the amount of small things you can see in a sprite. Readability is to do with how easily you can tell what is going on in a sprite. With sprites, readability is often higher priority than detail, especially for small sprites. In general, the smaller your sprite is, the more detail you have to forsake to keep your readability. Take a look at some sprites that have smaller and larger versions. Pokemon is a good example. Notice, as you already have, how the overworld sprites have much less detail. This is on purpose, because, to properly portray the character, they can't fit every little thing onto the sprite, otherwise it will look cluttered, things will blend into each other, and it will look a mess. Instead, they focus on the few key attributes of the character that allow you to recognise it, and see what is going on. That is what you need to try and achieve. I'll edit a Pokemon sprite to show you what I mean.
EDIT: Here. Here's a sprite from FR/LG. On the left is the original sprite. On the right is my edit. All I did was attempt to add in the detail found in the larger sprite. Notice that it is very difficult to tell what is going on in my edit. This is because there is too much detail, and it's become extremely crowded and difficult to discern. If you take a closer look at the original sprite, you'll see he has no mouth, no tufts of hair, no collar, no straps from his pack, no wristbands, no red-and-white shoes, etc. These are the things I added to my sprite. If you take those away, you don't lose the essence of the character - they aren't really important. Instead, they've taken the key elements of the character: Red shirt, blue jeans, and his hat, and used those. This allows for a far more readable sprite, but not a more detailed one. Mine is more detailed, but worse as a result.
Readability =/= detail. In fact, often they are polar opposites. Detail is to do with the amount of small things you can see in a sprite. Readability is to do with how easily you can tell what is going on in a sprite. With sprites, readability is often higher priority than detail, especially for small sprites. In general, the smaller your sprite is, the more detail you have to forsake to keep your readability. Take a look at some sprites that have smaller and larger versions. Pokemon is a good example. Notice, as you already have, how the overworld sprites have much less detail. This is on purpose, because, to properly portray the character, they can't fit every little thing onto the sprite, otherwise it will look cluttered, things will blend into each other, and it will look a mess. Instead, they focus on the few key attributes of the character that allow you to recognise it, and see what is going on. That is what you need to try and achieve. I'll edit a Pokemon sprite to show you what I mean.
EDIT: Here. Here's a sprite from FR/LG. On the left is the original sprite. On the right is my edit. All I did was attempt to add in the detail found in the larger sprite. Notice that it is very difficult to tell what is going on in my edit. This is because there is too much detail, and it's become extremely crowded and difficult to discern. If you take a closer look at the original sprite, you'll see he has no mouth, no tufts of hair, no collar, no straps from his pack, no wristbands, no red-and-white shoes, etc. These are the things I added to my sprite. If you take those away, you don't lose the essence of the character - they aren't really important. Instead, they've taken the key elements of the character: Red shirt, blue jeans, and his hat, and used those. This allows for a far more readable sprite, but not a more detailed one. Mine is more detailed, but worse as a result.