It's just used to place emphasis on. The soft mattress makes it comfy the warm blankets make it cozy|@sushimasters the. More than 100 answers from native speakers about natural usage and nuances of comfy.
Comfortable 的同義字@slato they both mean comfortable, but comfy is a cute informal way to say it this bed is comfortable. Q&a about usage, example sentences, meaning and synonyms of word comfy. Comfortable is the state of comfort.
Both are right, but comfy is more informal. Comfy的同義字@sushimasters the best way to understand this is to imagine a very comfortable bed.