Yup, it does narrow your market even further. But not in a very meaningful way. iPhone owners are drastically more likely to download and use your app than on any other platform.

The hard question is whether to launch on the web or mobile first. Once you've decided on mobile first, choosing iPhone first is a fairly obvious choice.

It's not always about reaching everyone as fast as possible. Going wide initially is usually a first time entrepreneurs mistake. Build something, figure out that your value proposition is compelling for your target users. As soon as you have hit "product market fit" go wide.

There is a reason companies are going iPhone first.

Your phone (iPhone or Android) is with you at all times of the day, which in a sense broadens your market.