I would assume the real reason they're getting a new ship (the Voyager-A?) instead of keeping the Dauntless is that the writers want a smaller, less-powerful ship so that the characters can be in greater peril when the S2 plot takes off.
The Dauntless isn't that big of a ship. It only has about 7 decks - in Prodigy, the Dauntless' MSD indicates 7 or possibly 8 decks (same or just a tad larger than the Arturis fake Dauntless).
It only appeared larger in Prodigy because most of the time we've seen it, we've only seen up close shots and those of people in relation to the vessel's size - which most of the time would make ANY vessel appear 'huge'.
Also, smaller doesn't automatically mean 'less powerful'. SF is (supposed to be) about efficiency. So they can easily make a seemingly small starship very much on par (or mostly comparable) with capital ships (power wise).
But, given that USS VOY had 15 Decks, I would surmise the VOY-A would be a similarly sized vessel.
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