Jewish practice is multifaceted. We look at the origins of customs, at their meaning and spiritual significance in our own times. We are concerned, especially at seder time with relevance and interest to children, and we also look to the traditional quality of "hiddur mitzvah" - making Jewish customs esthetically beautiful. So we have beautifully embroidered table cloths, ornate candle sticks, and carefully designed cups for wine.
These
cups embody many of these values. They are indeed beautifully formed,
in a shape bound to invite the interest of children - so why indeed are
they shaped like "an egg-and-a-half!"
The answer lies within one more facet of the Jewish life - the halakhah, the legal part of our heritage. Any legal system is interested not only in what should be done, - but also in how, when and how much! For example, philosophically it is enough to know that society requires that we pay taxes! In practice both the IRS and each family are interested in precisely how much should be paid by each person. We enter a bar and order a drink - and we know that the precise volume in a single (or double) scotch is carefully regulated.
We
drink four cups of wine, traditionally corresponding to the four promises
of redemption made by God to the People of Israel. So we must consider
how much (in volume) is considered to be a "cup" of wine. Here
we look at systems of measure used in ancient Israel. 
A "cup" was defined in the Talmud as being a "revi'it" - a quarter measure, which is itself defined as 11/2 (one and a half) "k'beytsa" units, where a "beytsa" is an egg.
Thus our cup is formed to look like "an egg-and-a-half!" in order to remind us of ancient units of measure. There are, of course, debates about the precise volume of an egg but we have followed the more stringent view associated with the definition "k'beytsa" which may allow for a small margin of error in pouring or drinking.
There are Talmudic authorities who take the view that each cup must be completely drained in not more than 2 swallows - and our shape allows help in gauging progress with each cup.
LE-HAIM! (To Life!)
Zvi Yaniv
December, 2004