"This bag is heavy."
Hiragana and katakana are used. Kanji is okay as well. a lot of people use katakana or kanji, I think. If you worry about them, I recommend that you type on the internet. If actually the word is used, I think that you can look the word as a candidate. ：D
Katakana can be used for emphasis.
So: "This bag is heavy."
Maybe the speaker was asked whether they were talking about the bag or the not cute heavy guy from earlier lessons. ;)
カバン is a loan word for Japanese, maybe borrowed from Chinese, that's why katakana is used.