Catapult's damage isn't based on weight, and it can't throw creatures
1.) Catapult's damage is based on the level it is cast at, not the weight of the projectile.
Although higher level castings of this spell will permit you to hurl heavier objects, that is not explicitly required to do more damage. Catapult states (XGtE, p. 150, bold added)
When you cast this spell using a spell slot of 2nd level or higher, the maximum weight of objects that you can target with this spell increases by 5 pounds, and the damage increases by 1d8, for each slot above first.
Although you are permitted to hurl heavier objects, upcasting this spell does not require a heavier object to do more damage. The spell specifies that you can throw a heavier object with an upcast version of this spell, but you definitely do more damage (whether or not the thrown object is heavier). By a strict reading of this spell's rules, you could throw a 1 pound object with a 3rd level casting of this spell, and it would do 5d8 damage (3d8 for the casting, +2d8 for the 3rd level slot).
As further evidence that the spell's damage is dependent exclusively on spell level, note that the object used by this spell when cast with a level 1 spell slot can be
one object weighing 1 to 5 pounds. (Ibid)
If weight determined damage, then the 5 pound object would presumably do five times the damage that a one pound object would. But this is not the case. The weight of the object does not determine the damage done by the spell. Only the spell slot used determines the damage: the fact that the spell slots also permit you to throw larger objects is incidental.
2.) Catapult throws objects, not creatures.
You mention throwing a 40 pound creature (reduced to 5 pounds via the Enlarge/Reduce spell) with the Catapult spell. This will not be possible as catapult permits you to throw objects, not creatures (see most recent quote). As funny as it would be to send a shrunken goblin flying with this spell, you'll have to satisfy yourself with smacking it very hard with a nearby object.
That being said...
There is explicit text in the Enlarge/Reduce spell indicating that causing a creature to grow to double its size will do the following:
The target’s weapons also grow to match its new size. While these weapons are enlarged, the target’s attacks with them deal 1d4 extra damage.
There is similar text indicating that shrinking will cause a target's weapons to do 1d4 less damage. That being the case, you might be able to talk the DM into letting your hurled small (5 pound) object which is returned to its true size (40 pounds) do an extra 1d4 damage, as implied by the text of Enlarge/Reduce. They would be under no obligation to do so, as the target of Enlarge/Reduce was not a creature (which is implied in the section on extra damage), nor is the object you are flinging with Catapult necessarily a "weapon." But I suspect a DM might be generous and let you have this one.
Especially since it would take two turns to set this strategy up, and it would require you to use both a first and second level spell slot. If you had just cast Catapult twice (once with a first level slot and once with a second), you could have done 7d8 damage total in these two turns (though not to two targets at once, as you're suggesting). Using your strategy, you'd do 3d8+1d4 damage (again, only to one target), which hardly seems worth it.