ok, very important thing is to know whatever it is turbocharged diesel or not.
turbocharged engine allows you to regulate boost, and it can change the torque curve significantly. there are low boost diesel engines with flat torque curve from 1500 to 3000rpm, and there are hi boost engines where torque rapidly spikes at 1700-2000 and fall down quickly after around 4000rpm. similar to hi boost petrol engines, only shifted around 2000rpm lower
I have seen dyno sheet (something like 2005 ford 2.2tdci 170hp) with maximum torque at 2500rpm, and maximum power at 3200rpm. official numbers was diesel standard 1750 and 4000rpm
for turbocharged engine the turbocharger is the main determining factor in shape of torque curve.
another important thing is, that SI engine is limited by fuel mixture it could ignite, knock resistance, and emissions it is allowed to produce (closely tied to mixture richness). that means, that power (and thus torque) is tightly tied to amount of air the engine can pump. so profiles of cams, size of valves, intake and exhaust manifolds etc. you can manipulate the torque by changing ignition timing, but you are limited by knock. knock is main problem in low revs, so this could be one factor in low low-end torque of petrol engine
CI is (was) not that much limited, and (in good old days) it could run air/fuel ratio anything from 10:1 to 50:1 . the determining factor here is the amount of fuel injected. so you could easily "mask" some fluctuations in amount of air sucked to the engine and by that flatten the torque curve. there is no knock problem here to limit the low end torque
also as somebody else said, you are working with much smaller rev range, so diesel engine is much closer to the "ideal point" (set by cams, manifolds etc)
I honestly don't know know about the contribution of compression ration, but I don't think it is big.
what I know is, that N/A diesel engines usually had much lower torque compared to the petrol engine of same capacity. last one I remember was VW 1.9SDi (CR 20:1) which has around 120-140Nm. at the same time, 2.0 petrol engine (CR 9:1) has maximum torque around 180-190Nm
next thing for diesel engines was fuel injection. for old VW PD engines injection was determined by cams and torque curve was limited by this. modern CR engines are much more flexible. there may be much more influence here (pre and post injection, maximum fuel pressure - atomization for high revs... ), I don't knowand yes, if you are not that interested in peak values, you can do anything you want with torque curve. either CI and SI
(I don't remember the numbers exactly, so...)
