Fu! Sion! HA!
Making their combination gigantic is some fan service. A smaller fusion that can cut giant things is a perfectly fine anime trope, and it wouldn't make us all think about Digimon. But it's fine, it's fine.
What's really important in these last two chapters is how all the characters come together through a series of one-liners. Especially Ruby and Lloyd apologizing (Wait, why is Lloyd apologizing?). It's a nice way to cap Ruby's arc. She might stay an arrogant troll, but she's proving her alchemist chops are the real deal and she won't be as antagonistic after this. Sometimes Solidarity means you have to work with awful people, because ape together strong.