Dialogue is highly artificed. It is not as similar to real-life conversation as we tend to think. In general, it is far more compressed than actual speech.
To make something smaller, you have to figure out what to omit. When I compress my sleeping bag before I pack my backpack, I remove much of the air. This air is essential for the function of the bag—it's what insulates me, holding the warmth of my body close while I sleep in a thin tent on a chilly evening in the mountains—however, air is also ubiquitous. I don't need to carry all that bulk in my back. I can get more when I set up camp, just by shaking out my bag. As a writer, you can also eliminate such bulk by removing the obvious, the assumed, the clichéed, the familiar. And there's a side benefit: what you jettison is easy to get back, but you've shifted the burden of supplying it to your reader, which gives the reader agency and a stake in the book. You don't have to say the sky is blue on a sunny day. Saying so is annoyingly obvious. When you allow your reader to envision that, the blue will be their blue. Maybe it's robin's egg, "One Morning in Maine," or cerulean to them. It probably depends on whether they're more into nature, children's literature or painting… Unless it's essential to your work, give them that choice, and you've made your pack leaner while simultaneously engaging your audience.
Now, let's shift the metaphor from camping toward atmospheric science, in particular to the air we squoze out of the bag. As an author, you should provide the oxygen and the noble gasses. Let the reader supply the 78% of the air we breathe that is nitrogen. Now, because the writer gave only dialogue but skipped over a physical description of a tertiary character who won't recur, the cook at the restaurant looks like my Aunt Sally to me, which either makes me love her or hate her…. Either is good. Compression begets agency. It's a win-win. This is true not just about dialogue, by the way, but also description.
When it comes to dialogue, it's useful to be hyperconscious about what we're leaving out and keeping in. Ask yourself: Should we hear from both (or all) interlocutors? Are you using quotation marks? How many, if any, assignations or dialogue tags? What about verbal fillers or stalling words such as "um," "like," and "well"? It would be easy to say that they're meaningless and unnecessary, but there are situations where the hesitation they imply is exactly what the writer needs to convey?
Greetings are often meaningless, formulaic and skippable, but it's important to realize that conversations about trivial concerns can also do a lot of work. Even though the spots on a water glass seem unimportant, the conversation about them could be the one that tells the reader everything they need to know about a marriage.
PROMPT: Take a passage of dialogue you've written and decimate it. Or better yet, cut it in half. See if it has more punch. You can always put things back if you miss them a lot.
Now take the decimation approach to any page of your book, and see if you can apply it there. Keep at this task of eliminating needless words till your prose is as tight as you can make it.