I've seen a lot of VS debates ended up arguing about how big or small Dragon Ball universes are. I don't claim to know everything, but I hope this clear some doubts so that we can focus on the actual debates.

Firstly, Dragon Ball writers, like any other writers, relies on intuition rather than paying attention to the physical correctness of works. Therefore Dragon Ball, like most fictional realities, should be assumed to either have different laws of physics, or that the nature of realities are not physical at all. Like how easily characters with mass surpasses speed of light. That is not to say using simple calculations are wrong, but too much reliance on laws of physics may have unintentional results or conflicts.

Secondly, this map. It was make back in the day when this is all there is for Dragon Ball, so the translations ought to be revised.

The entire sphere that was called "world" is actually the universe (uchuu, best and literal translation, no way around this). The "world" as coined by Whis, now refers to all 12 universes, and all 18 universes before the 6 was destroyed (sekaii, again, no ambiguity). What was "the Universe" is actually the mortal realm.

I've seen people saying that Dragon Ball is 3 universe big because there are also Heaven and Hell etc. Not true, it is as big as any regular universe. Heaven and Hell aren't exactly entire universes, they are realms, similar to the 9 (or 10) realms in the Thor comics. Some versions of the 9 realms are planets, some of them are smaller realities within the regular universe. Confusing.

Again we see "universe" in this picture actually refers to the mortal realm.

On the other hand, we have people saying that it is actually smaller, because it only contains 4 galaxies! Now the "galaxy" in this map actually means "quadrants" of the universe, which the 4 Kaios like King Kai looks after. We know this because at least in DB Super, we were shown many galaxies when Super Shenron shows up in Universe 6 arc.

Back when there wasn't Beerus to protect the Kaioshin, there use to be also 4 Kaioshin in the universe, but DB Super retcons them into just 1, from the fact that if he dies, Beerus dies too.

Lastly, something I didn't think I have to mention. Apparently some people also have the idea that DB universe is small from the scale of the map. Its a map. Even your world map isn't perfectly scaled, why would you have different expectation for a universe map.

But for now, at least we know that this map is still somewhat applicable given DB Super presents other universe as spheres as well.

Fun fact: there is actually no direct translation for "galaxy" in Japanese. "Ginga" is actually "Milky way". Correct me if I'm wrong because I don't know Japanese, but I can somewhat read kanji.