As I understand it, "Irrealis mood" is not a mood in and of itself, but rather a set of grammatical moods, which, according to Wikipedia "indicate that a certain situation or action is not known to have happened as the speaker is talking." I believe that your examples above of "than would be" may be examples of the conditional1 mood (one of the irrealis moods), but I admit a lower level of expertise in the subject of moods than I likely should have.
Nothing is being neglected, as these are both grammatical English sentences, but if one were to rewrite these sentences without ellipsis, one might rewrite them as the following:
- Programs are larger, more complex, or have a greater span than [it] would be practical [to contain] within a single project.
- The weather here is warmer than [the weather] would be expected [to be] at that2 latitude because of the influence of surface currents.
Obviously, as a sentence with much simpler structure (i.e. a shorter subject), the second sentence is easier to rewrite in this manner.
1 My original answer posited subjunctive, but upon further inspection, I believe it to be the conditional mood.
2 Incidentally, I would also change change that to this -- because we are not speaking of there, but rather here. Note: This was written prior to the question being updated.