Mushroomed,
At the risk of getting my arse jumped all over again,,, I will try to help you out a little.
The measurements you refer to are the diagonal measurement of the screen. A 40" wide screen is just about the same width as a 43" 4:3 screen(sorta), but bear in mind that the 4:3 screen is a lot taller. You will get a lot more viewing area on the 43" 4:3 screen but only when watching 4:3 material.
Earlier on this post I was comparing a 61" 4:3 to a 55" 16:9 as far as the viewing area of 16:9 material. This is the same thing, pretty much. I haven\'t gotten out a tape measure to figure out which size 4:3 screens were equivelent to which size 16:9 screens for widescreen material but this effect is perhaps what you have noticed. Hope this helps.