I don't mean to sound like a jerk but if you didn't know this before you bought the 27" iMac, you don't have enough experience/knowledge as a web designer.
I have never heard this comment before, anyone with 2 weeks of formal training would know the reasons behind your confusion.
Any web designer with a 27" iMac knows the benefits of the 27" screen is that you can look at your website with all your palettes and tools still onscreen.
Web design by essence, needs to be checked on multiple OSes, web browsers, and screen resolutions. I remember doing that 14 years ago when I worked for some start-ups.
The 27" is great for designing websites, you just have to understand the limitations of the devices used by the people intended to view your websites.