Well, the primary function of the bracket is to support the shelving. But, glass being such a transparent material is unable to hide anything, not even the brackets that support it. That's why it is good to have supporting systems that add to the beauty of glass. This continues to prompt designers to design brackets to enhance the grace of shelving. As such, it is not desirable to use brackets designed for glass shelves for supporting another kind of shelving.
Glass shelf brackets generally tend to be lightweight and delicate and in certain cases intentionally kept shorter than other types of brackets. It is understandable that a large bracket may dominate the glass shelf and thus present an uneven look to the shelving. Larger brackets are required when there is more of weight, be it of the shelving or of its contents. Glass shelves purported for display are not expected to be so heavy to call for larger brackets. It's only if you are going to use the shelving for storing of items that add to weight and necessitate a bigger bracket.
The nominal-sized glass shelf brackets are rails or clips. Such brackets hardly stick out of the wall, are often made of chrome, thus giving a glass like shine and naturally blend with the shelf that they support. These are first mounted on the wall, and subsequently the shelves are fitted onto them. No screws are driven thru the glass to keep the shelf fixed. In stead, they are provided with specially designed headless-screws that push down and grip the shelf in place. Their size is governed by the weight and thickness of glass that they are designed for. Rails and clips are recommended for glass shelves for keeping very light items and can't be used for display of heavier items.
There are many pages devoted on the subject of shelves of different design and materials, shelving accessories, brackets and much more at The Home Shelving Guide. The kind of information that is gathered there surely needs a visit by you.
If you have plans of keeping heavier items on these glass shelves and fear that this might cause sag, you should use only standard glass brackets. These brackets are designed to have more width than the width of the shelf. As such, the end that is not fixed into the wall will project out from the shelf but it ensures that all the base of the bracket is firmly supported. Thus you won't have to worry about sagging or depression of any kind.
These brackets have a raised foot on the end or an extended lip that firmly keeps the glass shelf in place. As we know that glass slides easily on the metal it is important to have a suitably designed bracket that prevents the shelf from gliding away.
There are a very large variety of brackets available for glass shelves. Depending upon your budget you may a choose one that goes well with the rest of décor, keeping in mind the kind of support your glass shelf might need.