@Kamran
As far as I know, diesel engines are limited in RPM by the combustion process. It is fairly slow by nature and if a diesel engine is made to rev, it might not actually produce any power due to "misaligned" combustion phases. Plus, (I think) emissions would be terrible. I'll have to check up on this though. I'll le you know ASAP.
It is interesting to note however, that the Audi R10 V12 TDi racer has a fairly high-revving engine, for a diesel that is, at 5500 RPM. They achieved this partially with a new common rail injection system that has much higher injection pressures than normal engines. The effective powerband for this engine is between 3000 and 5000 RPM.
Ghuncha
Basically, a diesel engine is always drawing in air without throttling, so you don't have the throttling losses associated with a petrol engine. This is only true at part load though. At full load, a petrol engine's butterfly is open 100%, so throttling losses become zero.
The second factor is compression ratio. Diesel engine compression ratios are always much higher than those of petrol engines. And by thermodynamic laws, cycle efficiency is directly related to compression ratio. In practice however, higher compression ratios in diesel engines necessitate stronger and heavier engine internals. This leads to other frictional and dynamic losses, thus diesel engine efficiency isn't as high as it could be...
For petrol engines, it depends on the type of turbocharging actually. There are new petrol engines available today that have high compression ratios, and small turbos. These turbo setups are designed to make a small engine give torque and power outputs like a bigger engine.
A few recent examples are the Fiat 1.4 turbo engine, the Renault 1.2 turbo engine, and the Peugeot / BMW 1.6 turbo. All these engines are designed for efficiency (more so the first two). That is to say, they have fat torque curves early in the RPM range, this reduces frictional losses since your engine doesn't have to spin fast to produce torque / power, frictional losses are also reduced by the fact that the engines are smaller. These engines aren't about outright power though, these engines are for flexibility.
Also, since these turbos are relatively low pressure, compression ratio is still relatively high. This again, keeps a high efficiency. Last but not least, the engine internals are very, very, robust. These engines are designed to run stoichiometric (14.7 : 1) air fuel ratios upto full load / RPM. Traditional turbocharged engines didn't do that. You had rich air / fuel ratios at high load, which is bad for efficiency and emissions.
Hope this clears up some points.
