1l versus 2.5l
" Engines, and in particular indirect injection gasoline engines operate most efficiently on full load and also have an emission minimum there."
Not only that but two engines with the same output would be expected to have exhausts and catalytic converters of the same size, because they produce the same amount of gas.
(Incidentally there is no reason, cf. above, why a 1l engine should be in principle less durable than a 2.5l one, if it is designed properly. Wear is mostly caused by metal to metal contact and with modern oils this is much less of a problem than in the past. Because linear dimensions scale as the cube root of the volume, the stroke of the smaller engine is only a quarter less than that of the larger one, so at the same piston velocity the boost pressure only needs to be about 0.9 bar compared to a normally aspirated engine. One the other hand the reduced weight means that superior materials can be used at lower overall cost. Higher combustion pressures give better thermal efficiency, warm up is faster with a small engine, so less risk of acid build up in the oil, and the surface to volume ratio of the combustion chamber is bigger making even heat extraction easier and reducing differential thermal stress. In a world of CAD and FEA, where detonation can be controlled, the smaller the engine the better.)