Of course, an 18k gold watch, with the same size as Apple's, that contains more gold is heavier by definition. I wouldn't be surprised if that factored into the decision.
Weight is critical to the iWatch. Look at the advertising they have done to date. It has all been in women and men's fashion magazines (with a particular focus on Asia) whose readers will generally have slighter frames.
The iWatch is quite different from other watch makers in this respect since they are targeting both men/women with the widest age range probably ever seen in the industry. This means the same watch needs to be capable of being worn on a 13 y/o Chinese girl as well as a 35 y/o 6 foot American guy.
