Somehow we have come to use 'massive' somewhat colorfully to lend a sense of great magnitude. You shouldn't see that in a science journal. It can feel a lot like slang when it gets paired with an insult that is entirely unrelated to mass, as in 'He is a massive traitor.' But it is very common to (unscientifically) talk about someone's massive heart attack.

I use it, its not slang.

Yes. It's a legitimate word. It's very slightly technical. It can refer literally to mass = weight = kilograms or tonnes. It can also be used to mean "a lot." It is used in a straightforward way in slightly formal or technical contexts. Literally, referring to mass, "Jupiter is a massive planet." In the sense of "a lot:" "The US government is spending a massive amount of money on home Covid tests." Because of it being a somewhat elevated or technical word, it's sometimes used in a half-joking way. You are using a serious word to describe something silly or unimportant. For example: "This is a steak restaurant. Don't order fish here. That would be a massive mistake." "Prince Harry is a massive headache for Queen Elizabeth."

Massive is not slang. It is an actual word that means... well, *massive*. When we use it colloquially, it is often an exaggeration of something large. A lot of times, something massive is just that, massive, but can be subjective depending on how big something should be in comparison with something normal. Examples: He has a massive nose. (Larger than normal, but not exactly a nice thing to say.) Jupiter is massive compared to Earth. (Jupiter is massive, so this is quite true.)

I personally use it literally, in conversational English, to describe something that is really big. I have rarely heard it as slang.