I know that one is an i-adjective and the other a na-adjective. But what’s the difference in meaning? Or is it purely a grammatical difference.
I noticed that most of the time, when there are multiple words with a similar (if not identical) meaning, there is a difference in connotation still.
でかい and 巨大 are rarely interchangeable, because they differ in both register and scale.
でかい, though standard enough that I wouldn't necessarily call it "slang", is a strictly casual expression that you would be extremely unlikely to see in formal or official contexts. In terms of scale, it's not much different from the standard 大きい, perhaps a little more emphatic.
巨大, on the other hand, is quite neutral in register, if anything a little more towards the formal than the casual end of the scale (as is usually the case with 漢語 versus 和語). It is, however, much more extreme in terms of scale, indicating that something is truly overwhelmingly large. As such, it's actually also quite unlikely to appear in many formal/official contexts, simply because formal texts tend not to be hyperbolic. It could certainly appear in the likes of newspaper articles or prose descriptions in novels, however, whereas でかい would be less likely to appear in these unless they were written in a particularly casual style.
To draw parallels with English words relating to size, I'd say:
大きい ≈ big/large
でかい ≈ huge
巨大 ≈ gigantic/colossal
