In general trying to lighten a bicycle isn't particularly cost effective. Bike parts are much more expensive bought piecemeal than bought as a complete bike. But is you insist the best way to go about is is to dissassemble your bike, weigh the parts (seat post, saddle, cranks, forks etc etc) and then replace them one by one with lighter parts.
NOOOO! Do not make that mistake!! A lighter bike is a better bike. I own a Specialized FSR Comp that weighs about 30lbs, but I'll probably lighten it. Lighter bikes are much easier to uphill as well.
bigger cyliders, bore out the engine, make it lighter
You replace part by part with lighter and more expensive parts. usually not a particularly sensible thing to do as bike components bought piecemeal are much more expensive than when bought as a complete bike.
bore out the engine, make it lighter, and add nos at bis racing.com
Simply change the exponents on the bike for lighter ones, like the wheels, tires, seat post etc. this will cost more.
if you want it to go faster with no performance parts you will need a lighter person
A street bike is a type of BMX bike, so your question can't be answered.
As a general rule if two bikes are in the same price range, the smaller of the two will be lighter. This because there is less material used in the smaller BMX due to it's much simpler construction. But, when you head for the high end, a flatland or racing bike is going to be much lighter compared to a mountain bike.
There's no telling. The BMX is probably smaller, so it should be lighter. OTOH they're often amazingly chunkily built and can be surprisingly heavy.
Titanium is a lighter metal than most others. Bike manufacturers use titanium to make a bike go faster.
You start off with a light frame, then you get a light fork, light wheels, light tires, light bars, light seat post, and light cranks. To get a light bike, you really have to use light parts through and through, and it's cheaper to buy a light bike already to start with than to lighten a heavy bike piece by piece.
Basically, Titanium is stronger. This means that you can use less of it and still end up with a lighter bike that is strong enough to ride.