My elder son has trained under two olympic coaches who told him, in the context of competition, one pound on a bike equals seven pounds on the body.

For those of us who ride on the street, this kind of lightness doesn't have that much relevance.

That said, I have found, in general, that the more mechanically-refined (more reliable) parts on fixed-gear bikes tend to weigh less.

They don't weigh less because of their mechanical-refinement, but because the designers have figured buyers of these parts want both lightness and reliability, and so they go together.

In contrast, I have just had a fixed-gear mountain bike made for me by Webcyclery, and I note on mountain bike parts, the manufacturers tend to focus more on reliability than on lightness, and so my fixed-gear mountain bike weighs considerably more than my fixed-gear street bike.

When I pick up the two bikes and compare them, I can feel the difference.

However, riding them, I really don't feel any difference I can attribute to weight.

In fact, I actually like the mountain bike a little better (even though it weighs more) because I knew more about fit when I chose my mountain bike components than I did when I put together my street bike (it fits better).