There is something to be said for understated elegance, and subtlety is also a good decor element in certain homes or particular spaces, but bigger can be better in large rooms. Spaces with extra high ceilings are a good example. They need grand elements to balance things out, and rooms with large areas of blank wall need large pieces or groupings to keep items from looking lost against the backdrop.
While you would not typically use oversized pieces in an average space, they are actually necessary in a large room. Large plants, trees, large furniture and artwork pieces, and even large lamps and accessories will bring the space together better than smaller items will. If the furnishings and accessories are too small, the space feels cavernous, empty, and cold.
To warm up a large space, another trick is to avoid pushing furniture up against the walls. Bring pieces more toward the center of the room, or break the area up into sections. Anchor the seating area with a large colorful rug, to draw the eye away from vacant space around the perimeter of the room. Placing more pieces into the room is also helpful, such as a sofa table behind the sofa or loveseat, which you can adorn with greenery or use to add more accessories.
Artwork should be hung higher than usual (above eye level) in a room with a high ceilings. You can also create an illusion of less height by raising your curtain rods higher above the window and using long panels that reach the floor. Consider painting the ceiling a darker color than the walls are (or have it painted by a professional if it is extremely high) to make the ceiling seem lower than it really is. You can also try painting the lower half of your walls one color and the upper sections another color, to break up the height and help make a grand space still look grand while feeling less imposing.