When shortened forms become idiomatic, it may be better not to try to identify parts of speech within them.
I had more cups of tea than Harry [had] [cups of tea]. No problem with
classifying Harry as a noun (phrase) (though 'they did' reduces to
I had more cups of tea than [I] [had] [cups of] coffee. Again, no
I had more money than [proved to be / was] necessary. No problem; adjective.
I had more cups of tea [this morning] than I had yesterday. Temporal adverb.
I['ve] had more cups of tea than yesterday. Is this an ex temporal adverb? A temporal adverb modifying a missing verb?*
This becomes far more ambiguous when one tries to recover the undeleted form of say
It's warmer than yesterday. ... It's warmer than it was yesterday or It's [a] warmer [day] than yesterday was?
With 'more ... than usual', it can be very hard to determine the undeleted original (which may not have been used in particular cases anyway). This means that attempts at logical analysis are going to be unconvincing.
As CalifJim says on EnglishForums:
'[more X than] usual':
This is a fixed phrase. There is no [apparent] logic involved.
With simpler forms such as think big, drive safe, work smarter, take it nice and easy, adjectives are being used (perhaps unconventionally – often for emphasis) as [if they were] adverbs. The acceptability of this practice, and other considerations, have been addressed here before. Look up flat adverbs, eg at 'Using short adjectives as adverbs, such as “easy” & “short” '.