Yes, but you forgot transmission losses if we take into account transmission losses diesel-electric hybrid engines are more efficient. All those gears and shafts mean big energy losses for diesel engines.
Yes (specifically), but in general depends.
Thats why i mention "holding all else constant" (i.e assuming downstream system losses and many other issues too are the same) and look at just energy conversion loss (i.e thermodynamic loss) for the immediate intuition that timur had and pointed out.
But these factors are of course anything but constant when you talk about vastly different sized engines, torques and power ranges....and typical operation profile of the said engine.
The transmission losses would indeed be higher for larger (wheeled) propulsion systems....as mechanical transmissions commensurately grow to handle the vast divide between what the load at wheels can be compared to the engine optimal operating range.
The even larger issue (for very large IC engines ---> tractive wheels) is in fact the inevitable mechanical transmission design complexity, resulting size+weight and expense to begin with given the lowest gear tractive effort needed especially for locomotives which have large loads needing to be pulled external to its own mass.
But it is not the universal case as generator, switchgear size (weight and volume) become severe penalties for smaller IC engine propulsion alongside the conversion loss. Which is why (a now smaller more elegant) mechanical transmission makes much more sense for those generally.
In between clear big and small...there is assortment of factor analysis to be done for optimal feasibility, efficiency and performance.