There are many Google Cardboard viewers, but the GoogleTech C-1 Glass is one of the most compact viewers.
I already have a cardboard viewer but I found it bulky, and I could not tap on the screen without reopening the viewer. I liked the elegant design of the C-1, so I ordered one from Google Store.
The C-1 Glass comes with a dustbag, not a hardcase shown on the company's website.
It is as compact as I hoped - about the same size as a pair of eyeglasses. Here it is next to my old cardboard viewer by Ritech.
Compared to a typical viewer like the Ritech, the C-1 is much more portable. However, if it is sunny, I can sometimes see the reflections of light on my face. Another issue is that the C-1 doesn't have a head strap, so it must be held up while being used. It wasn't too tiring for me. I just rested my elbows on my chest.
If you're looking for a reasonably-priced, compact Google Cardboard viewer, and you don't need a viewer with a magnetic trigger, the C-1 is a very good option.