Cherry, while you're offering good points, I should mention that each Gundam is different depending on the timeline (and even when in the timeline). For instance, the fictional alloy Gundanium can only be made in stable gravity spots in Earth's orbit, where the Space Colonies are found. This alloy is what allows a Gundam to move and act like it should (really, really, really high-performance Mobile Suit) while still being as strong as it is.
If my memory serves right, Gundanium is the step above Neo Titanium, which is twice as strong (and heavy) as regular Titanium. Gundanium, however, is more like four times the strength of regular Titanium and something like half the weight... or I could be off on the weight.