A neutron star, as small as 12 miles in diameter but containing half

To see why (D) is problematic, it might help to consider a simple example:

Mike ate as many pancakes as Dana.

Makes sense, right? The sentence is comparing actions, but the second is implied. Mike ate as many pancakes as Dana [ate.] This construction is fine.

But watch what happens when we introduce a possessive:

Mike ate as many pancakes as Dana's.

The possessive makes it sound as though we're talking about Dana's pancakes, so the sentence would be identical in meaning if we wrote "Mike ate as many pancakes as Dana's pancakes." But notice that we're still comparing actions -- how much one entity ate to how much another entity ate. In other words, we appear to be comparing how many pancakes Mike ate to how many pancakes Dana's pancakes ate!

Now, it might be true that if pancakes were conscious entities that needed food, they'd choose to eat each other. But on the GMAT, that's clearly an illogical construction.

Similar problem in (D). Here's the relevant portion again:

.
The possessive here gives the impression that we're talking about the sun's matter. But a neutron star doesn't contain as much matter as the sun's matter contains! It contains as much matter as the sun itself contains. So the possessive creates an illogical comparison here.

Moreover, if you weren't sure about the comparison, there's the problem that "but containing half as much matter" isn't parallel to anything. Either way, (D) is out.

I hope that helps!