Ok I am knew to this sport in regards to you (well a lot of people) but to answer all of those questions simply I will say this
Since 1992 the sport has become heavily segregated. In 1992 (from what I have seen and heard) you guys would do 'dh' and xc on the same bikes. Now there are 2 (possibly 3) distinct sectors of the sport. Xc is still the way you describe. Light bikes, even suspension is light, lockouts etc etc...speed under full manpower all the way. Dh today is signifigantly different from those I have seen in old mtb movies and quite a bit more technical (strictly from talking to old dhillers and watching old vids) The sport itself has evolved so the bikes are heavier, tires bigger and more suspension. However for a short history lesson, suspension has shrunk. We are actually seeing a decline in the size of suspension as I believe companies kind of found a sweet spot and are developing better shocks instead of suspension designs. We are also seeing dhillers shrinking their tires. For a short time 3" tires, 9 in rear and 10 in front was the golden bike. But we have moved backward to the point where it isn't unheard of to see shorter more 'efficient' forks on the front, shorter travel shocks and lighter built bikes in general.
Ironically, big boy forks (there are still a couple for the big boys and guys who go HUGE)are based on motorcycle tech. The original monster T's stole EVERYTHING from motobikes. The cartridges and size. But I would say a very small number of people use them and a smaller number need them. But yes...these do look like motor bikes
Speaking of roll down in size, I do believe we are seeing a roll back in need. Freeriders for a few years were trying to break the 50 ft mark for a drop (not succeeded) and a few were hitting 40's consistently. I believe that this has toned down and riders realize 40ft is just about the limit, so now, riders are dropping the size to a reasonable 30 ft (hahahaha reasonable sheesh) and doing tricks like bmxers. Backflips, barspins, can cans etc etc...
1) mountain bikes have gotten heavier and "heavy duty"?
Yes, but only in dh and fr. Xc is still light and lean. Trail bikes range from heavy xc to light freeride bikes (the spanner between the two extremes) If anything suspension is lighter than ever and more efficient making todays bikes better in performance than older xc race bikes. But, to clarify, I don't do much xc and even doing xc I ride a big bike so I really don't know much beyond heresay on that topic specifically.
2) the tires are bigger and fatter (extending more outwardly from the rim)?
Better traction at the speeds of dh, better traction for freeriding especially on shore stunts, but not entirely as xc tires are still skinny
Endless supply of reasons....less air needed etc...
3) they overall resemble motorcycles more now than they did before?
Depending on the setup I can't disagree. My gf's bike setup as is looks like a motocross bike with no motor. Can't deny the look but we are moving more towards the size and style of MX riders and away from xc riding. We still (unfortunately) inherit many things from the old school xc guys. Small headtubes and small bottom brackets are a battle engineers have been trying to beat for the last 5 years as the sizes used on road bikes are tiny and weak not allowing for bearing sizes to except the loads given out by current day freeriders and dhillers.
Well...that was long. Cheers
