I've seen huge junipers in bonsai back yards that just don't look like anything but a tree in a box.
A huge tree in a box is just a huge tree in a box... just like an oil painting before the application of skill is just a bunch of paints, brushes, and a canvas.
Bonsai is an art form. You can have huge art, or tiny art. But at the end of the day, you are seeking to apply skill and design to create something that is appreciated for its beauty and emotional power. Bigger isn't always better and a big bonsai isn't necessarily more valuable than a smaller one. It all depends on the quality of the art. Size, in and of itself, doesn't matter.
After a while, I believe the size of the tree starts to work against you if it is too large. The cost of maintaining, potting, repotting, styling and moving those trees starts to become prohibitive, and can place them outside of the realm of most people. You can still have a poor bonsai even if it is large... but it would make me cry if the cheapest pot I could find for my poor bonsai cost $3000 and required four people to move