Below the frequency where radiation from the driver becomes omnidirectional baffle size will have zero effect (except on diffraction of higher frequencies). If i remember right this is the wavelength equivalent of the cone circumference.

Therefore bass from driver 'X' in a slim baffle will be exactly the same from one in a wider baffle, Martin is correct. At low frequencies it's the amount of air you can shift that counts

Go & purchase High Performance Loudspeakers by Martin Colloms & have a good read. I have the latest sixth edition which i recommend for debunking a good load of cobblers