In default CC3 overland fantasy maps, the drawing tools (like for forests) and symbols (like for mountains) don't "scale" between canvas sizes because they are not supposed to be a 1 to 1 mapping (where one tree equals one real world tree or - perhaps more importantly to grasp - one mountain symbol equals one real world mountain).
Now if you were to create a dungeon map at 1000 x 800 feet and another dungeon map at 100 x 80 feet, if you draw a floor for a 30' x 30' room using a floor drawing tool, the square floor would look smaller in the bigger area map and larger in the latter map because those are "scaled".
Now there are ways to make the drawing tools output larger symbols or fill styles, but I'm not confident enough in that to risk explaining it since changes to a drawing tool effect all maps, not just the one you are working on. If you want a larger default symbol universally in an overland map, go to File -> Drawing Properties and in the Drawing Presets dialog box, make sure the Default Symbol Scale box is checked and type in the new "scale:" beneath it (the default is 1.000). Note this will not effect the drawing tools. And if you want to change the scale of bitmap (PNG artwork) fill style (won't effect drawing tools though) or the size of a vector symbol used in a symbol fill (will effect drawing tools using vector symbols for that drawing only), see my instructions for both in this recent thread:
Adjusting floor texture in DD3