From what I understand, the low power to weight ratio of diesel
engines has been the main limiting factor for their adoption into
small planes (and passenger cars for that matter). The reason for the
high weight as it was explained to me was that the high compression
used for an auto-ignition type engine required strong, and thus heavy
castings to withstand the pressures.

So these days you see diesels fitted with forced induction systems,
which apparently makes them more peppy (e.g. the 90hp VW 1.9L TDI).
But doesn't this more powerful charge being introduced by the forced
induction system, just require again, a heavier engine to withstand
the more powerful explosion? Or to put in in converse, if the same
engine could have withstood the more powerful charge, couldn't they
have built it lighter in the first place and used a conventional
induction system? For now lets ignore the altitude power
normalization aspect for aircraft operation.