Why didn't they build narrow hallways and doors so that no dragon would fit in?
Dwarves are short and stouty, so why didn't they go "Tucker's Kobolds" on Smaug?
According to the Tale of Years in Lord of the Rings, Erebor was founded in TA 1999 but dragons didn't reappear until TA 2570 (Smaug himself didn't descend on Erebor until TA 2770). No dragons come into the tales of the Second Age.
So that establishes a timescale of about 6,000 years between the previous appearance of dragons in Middle-earth (at the War of Wrath) and the reappearance of dragons in the Third Age, and establishes that Erebor was founded close to the top-end of this timescale.
From there it's obvious - as far as anyone was concerned dragons were an extinct creature that only appeared in stories from the Ancient World. There hadn't been a dragon seen in Middle-earth for about 5,500 years at the time Erebor was founded.
The simple answer is that the dwarven halls of Erebor were meant to be palatial. They were the residence of a king and other noble individuals and were a display of great wealth. Tolkien's European sensibilities couldn't imagine anyone building sneaky little rat tunnels and tiny abodes unless they were poor, unless they were peasantry.
If you were to somehow visit the Queen of England today, walking into Buckingham Palace, you might think to yourself "this reception hall is big enough for a dragon to fit into". You live in a home where the highest ceiling is what, 9 or 10 ft tall? There would be plenty of rooms with ceilings double and triple that height, maybe more. Your home has doors at most 48" wide. But again, in the palaces of royalty, doors that are more than double that width are quite common.
How could Tolkien have envisioned Erebor other than as described?
One other consideration that might be worth mentioning: These halls were not for just people, but for horses and wagons too, and were main traffic routes in their city. A dwarf mountain is an entire city, not just a single palace. It would seem logical to have wide halls as a way to accommodate the massive amount of traffic through them.
Might it be that the dwarfs mined the mountains and re-used the mined area's to live in. The great hallway was once one of the rich finding places and as such mined to the full extend.
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