The martini glass is a subset of bar glasses designed for specific purposes. Despite common belief, the martini glass is not the same as the cocktail glass. The cocktail glass is smaller, more rounded with a narrower rim and a narrower and shorter stem. It also predates the martini glass by about fifty years.
Though the martini glass design may look arbitrary and somewhat of a balance challenge, there is a true function to this elegant and unique drinking vessel. The long stem is a design element that allows for holding the cocktail so the heat of the hand doesn't cup the bowl and warm the drink. This feature keeps the martini cold so there is no need to add ice which would dilute the drink.
The conical bowl is a functional element that aids in the enjoyment of the gin. There is a tale that the wide mouth was designed to allow for quicker dumping of the “illegal” alcohol in the event of a raid on the speakeasy during the Prohibition era, but it’s more likely that the wide mouth allows for more surface tension which enhances the natural bouquet of the juniper berries in the gin.
Though larger than the cocktail glass, the traditional martini glass was originally designed to hold a two to five ounce cocktail, (about the same volume as the cocktail glass). A martini is all liquor and totals only 2 to 4 or 5 ounces of liquid*, it is meant to be sipped not gulped, so the advantage of the smaller volume glass is it keeps your martini cold while you drink it. These days "designer" martinis require a larger capacity because of the addition of juices and mixers. It’s not uncommon to see "martini" glasses that will serve a six or even eight ounce cocktail.
Some of my hand painted martini glasses.
Martini glasses now come in all sizes and shapes and configurations. There are hand blown glasses with imaginative stems, etched crystal glasses and fun, colorful hand painted glasses available today. There are “inverted” glasses and even martini bowls without the stems that come nestled in a larger fishbowl shape you can fill with ice. I have a giant martini glass that I use for chips, flowers and party favors on my buffet table and there's even a cat tree in the shape of a martini glass, but the one common design element is that distinctive conical bowl with the wide mouth.
* The volume of a cocktail will change depending on how much is poured but the method of mixing and how long the cocktail is mixed will also affect volume. A shaken cocktail will end up higher in volume than a stirred or thrown drink. The longer you stir or shake a cocktail the more dilution, thus more volume, occurs. The amount of ice in the mixing vessel will also affect end results.