I saw commentary on this piece in today's NYT on a few sites today. It makes an interesting argument: the rise in average house square footage (or, in other words, the relative ease with which a give amount of space can be had by the population, on average) has provided the means (not sure if they are going as far as to say 'an incentive') for American families to have more children.
In most developed countries (of the OECD variety), fertility rates are markedly lower than in the US. The cost of housing is also much higher, making it difficult for young couples to afford much space in which to raise a family...possibly leading to having fewer kids. One big example would be Italy; housing is markedly more expensive (not so much housing stock to go around) there and the economy is not exactly going gangbusters...which has led to -- get this -- 82% of men under 30 still living with their parents. Talk about mommas' boys! They also have an abysmal fertility rate: 1.29. In comparison, the US is at 2.1 (roughly replacement rate).
UCLA prof Matthew Kahn is quoted too (his blog is worth reading), with a different argument: so many of these newer, high square footage homes in the US are in outlying areas (where land is cheap), leading to longer commute times, serving as a drag on the return to the workforce of women post-child...leading to more of them staying out of the workforce once they get out, and having more babies.