"Over scaled" means that the map is too big. It takes too long to get from a certain location to another. Take a look at some official maps in game. Run around and see how long it takes for you to get to a certain point. For example, since you are making a KotH map, load up Viaduct and see how long it takes for you to run from spawn to the control point. Usually it's good to have a nice understanding of a gamemode and how it plays when your map for it.
Also, download some of the decompiled Valve maps and load them up into hammer. Decompiled maps can be found here:
https://tf2maps.net/threads/valve-maps-decompiled.5952/ . See how big some areas are in terms of units. Compare that to your own map, and maybe try to make the sizes of your map more similar to that of the official maps. You should also check out the TF2 Mapper's Reference:
https://developer.valvesoftware.com/wiki/TF2/Team_Fortress_2_Mapper's_Reference . In this, you can see class speeds, jump heights, and many more. It's not a bad idea to have this bookmarked in your browser for quick and easy access.
Also, I understand you want it to be industrial themed, so it "needs to be big to fit big machinery", but you have to be careful with letting your theme determine your layout. Don't feel limited and forced to have certain layout aspects (especially if they are bad) just because the theme says so. Map a layout, and then when you get to detailing determine how you want to fit that in. (It's not terrible to have a rough idea how you want to detail an area even if you are still in alpha, but don't force your layout to fit that.