better word for "fat"

[Hardwork]

Any better word for "fat" in these sentences?
THE CAPSULE IS SO FAT.
The book is so fat.

[Dozy]

Assuming you mean the book has many pages, "fat" is possible; it feels like a "colourful" description. The plainer and more standard adjective would be "thick".

I'm not sure in which sense you are using "capsule". I'll assume you mean a capsule containing medicine that you have to swallow. "fat" is possible and again feels "colourful". In this case "thick" doesn't seem quite right. The only other options I can think of are general-purpose words like "huge", "big", "large", "enormous".