The analysis of comparative constructions where the second comparator seems to b...

The entrance is covered with fat stones, and is narrow, because it was necessary that the stones which lie over it, should reach from one wall to the other; yet, strait as the passage is, they seem heavier than could have been placed where they now lie, by the naked strength of as many men as might stand about them. They were probably raised by putting long pieces of wood under them, to which the action of a long line of lifters might be applied. Savages, in all countries, have patience proportionate to their unskilfulness, and are content to attain their end by very tedious methods.

From A Journey to the Western Islands of Scotland

In revealing people's strategies for handling success and failure, the book demonstrates that the capacity to change across entire lives is much greater than used to be believed was true.

From a blurb about Ambition: How We Manage Success and Failure Throughout Our Lives by Gilbert Brim

In the two comparative clauses with than,

• they seem heavier than could have been placed where they now lie
• the capacity to change across entire lives is much greater than used to be believed was true

what exactly has been ellipted in the comparative structure, and how would you reconstruct the full, unellipted form?

Here is my research on the two comparative structures.

I think the answer to the first question is that they seem heavier than the stones that could have been placed where they now lie seem [gap/trace].

And the answer to the second one, which I derived from my chat group, is that the capacity to change across entire lives is much greater than it used to be believed that it was true that the capacity to change across entire lives was [gap/trace].

I'd say the elided parts of those sentences would best be interpreted as a noun (and any relevant pre/post-head dependents) + relative word, as in

• they seem heavier than (any stones that) could have been placed where they now lie

• In revealing people's strategies for handling success and failure, the book demonstrates that the capacity to change across entire lives is much greater than (the capacity to change that) used to be believed was true.

Constructions like these are touched on briefly in The Cambridge Grammar of the English Language p1121, tagged as rare and of questionable grammaticality as they cannot be created systematically - though I would not have tagged either of the examples in the OP as ungrammatical.

Consider also the following, where the AdjP in the matrix is predicative rather than postpositive:

[45]

i This result is better than __ would probably be achieved by a vaccination policy.

ii The price was higher than he wished to pay __.

iii When children start school they tend to get books that aren’t as rewarding as they’ve had __.

iv The eastward movement of the Atlantic thermal ridge was forecast to be a little less than __ actually occurred.

[...] the missing element from the comparative clause must be understood as an NP, not an AdjP: they are equivalent to ‘. . . than the result that would probably be achieved. . . ’ , ‘ . . . than the price that he wished to pay’, ‘. . . as the books they’ve had ’, ‘. . . than the movement that actually occurred’. It is questionable whether such examples are frequent and systematic enough to qualify as grammatical; certainly the construction illustrated here is not generally permissible, as is evident from the clear ungrammaticality of *This candidate was much better qualified than they appointed (“than the one whom”), and the like.