The reason is absolutely one of design and production capabilities and "hey look at this fancy thing, come give us your money". I am going to go out on a crazy limb that you already know that and are pointing out that it's stupid, which is entirely correct. But nonetheless...
1. Big things seem to sell pretty well. Not just "crazy named stuff that holds important lore and whatever", but just straight up "is actually big". Knights sold out faster than anticipated for their first run, and part of that was the whole "It's a Lord of War class thing that you can take squads of outside of Apocalypse (if you can spare the points), has great stats" and part of it was "I remember Knights from Epic!", but a big part of it was "Look at the fucking size of that thing. Look at all the parts on the sprues, you will get erect assembling that."
2. With their design software and production facilities, they can make bigger shit than before, but also they can
sort of make fancier shit. There's actually some stuff they can't do as well as before (from what I understand, having small studs all over armour is actually a huge pain in the ass for them), although "big bursts of entangled flames and wavy shit" is something that they obviously can do, as seen in half the fucking content lately. See:
Ynnead,
Lord of Change,
Exalted Flamer,
Burning Chariot,
Brimstone Horrors,
Old Shaggy Naggy,
Death Lords,
THIS FUCKING THING I MEAN SERIOUSLY WHAT THE FUCK,
Spirit Hosts, and of course
this fucking mess.
Overall the CAD software has some very solid upsides, but there is too much wavy shit going on with all the ghosts and flames and odd shaped feathers and everything. But yeah, they're basically playing to their strengths of what the software is good at and allows.
3. It wouldn't surprise me TOO much if they want Rowboat Gullyman to be bigger so he's "easier for kids to paint". That's kind of cynical from my part, but it would fit their general "sell Spess Mehrins to kids" MO.