How do I say something "contains a lot of content" using a single word?

I would use comprehensive especially in a positive sense

complete and including everything that is necessary

A number of words might be suitable, depending on the context. They include copious, abundant, lavish, rich, extensive, ample, overflowing, bountiful and profuse.

The text, article or blog post is said to be informative.

If you want a word that is unambiguously positive, use informative. If you want to emphasize quantity a bit more, and don't mind a possibly-negative connotation, you can use dense or maybe packed.

You can say that something is bloated, although it often expresses the concept with a somewhat negative connotation.