I think it's important to remember that Molly Weasley wasn't a fan of enlarging charms, so it may have been that The Burrow was made only as big as was possible through plain old manual labor. (Note: Mrs. Weasley doesn't seem to be opposed to using magic at all when it comes to home construction -- Harry notices, when he first arrives at the Burrow, that magic seems to be holding the Burrow upright and keeps it from falling down in on itself).
Harry couldn’t see how eight people, six large trunks, two owls and a rat were going to fit into one small Ford Anglia. He had reckoned, of course, without the special features which Mr Weasley had added.
‘Not a word to Molly,’ he whispered to Harry as he opened the boot and showed him how it had been magically expanded so that the trunks fitted easily.
Chamber of Secrets - page 53 - Bloomsbury - chapter 5, The Whomping Willow.
Some may ask, "Well, what about the magically expanding tent the Weasleys used at the Quidditch World Cup?" I think it's important to remember that Molly didn't go to the Quidditch World Cup in Goblet of Fire; she stayed home at the Burrow. As well, the tent the group used at the World Cup wasn't even the Weasleys' property -- Arthur borrowed it from a co-worker (Perkins, who, unfortunately, had been rendered unable to camp much, due to a magical malady called lumbago) and even then the tent wasn't super big. It had three modest bedrooms, a kitchen, and a bathroom.
The Weasleys took pride in their few precious possessions (for example, the Weasley clock and Molly's brother's watch that she ultimately gave to Harry for Harry's seventeenth birthday -- in the book, JKR makes sure the reader knows the watch is well worn and a bit beat up). So, while the Weasleys may have been able to use the extra room an enlargement charm or an undetectable extension charm might bring, one might argue that the Weasleys would reject utilizing magic of this kind -- they are proud of what they have and may not feel like they needed to use magic to appear wealthier than they really are.
And it really may not have to do with perceived wealth. Molly Weasley uses a lot of magic in cleaning and organizing (see chapter 6, The Noble and Most Ancient House of Black in Order of the Phoenix -- well, most of the chapters in Order of the Phoenix that take place at Grimmauld Place feature a lot of cleaning spells and magic) -- perhaps she simply didn't feel she needed a larger home.