I think your issue isn't the placement of things, but rather the perspective. It will definitely feel smaller when you're viewing it at the same eye level.
Try doing it from either a worm eye or bird eye view instead. Here's a few I sketched just now:
Sorry if they're not very good, but anyway...
On the left drawing(the worm eye view), I used tall buildings and put them right at the foreground to show that they're only that big, even up close. Since perspective dictates that the farther an object is, the smaller they get... placing a big volcano in the background suggests that it must be massive despite supposedly being smaller from the distance you are viewing it. Pretty sure that was what Ficcoon meant earlier as well.
On the right drawing(the bird eye one), I mostly did the same, except I took the buildings from the foreground and put them on the background together with the volcano. Together with the small clouds and tiny buildings, the volcano on the illustration appears gigantic in comparison. In short, angles other than a plain mid-angle view work better for amplifying the scale of something.
Hope this helped somewhat. Your illustrations are pretty nice though -- I really like the aesthetic of the trees in your original ^^
I think it's cropped too close, and that makes it feel kinda claustrophobic. You'd think that would make it feel huge, but it seems to remove a lot of the context from the explosion, making it seem just kinda... squished.
I'd probably do a search on "huge explosion anime" to get some ideas about how to make it look epic. Anime is great for showing unreasonably large explosions.
I don't have much experience with explosions, but I know a good trick for making things look more massive is to turn it to 3-point perspective. In case you don't know what that is, simply make the base of the explosion larger than the top of it, as though you were looking up at it and seeing it narrow into the distance.