fox news sunday host appeared on the mike gallagher show on friday .
said mom-of-one clarkson ` could stay off the deep dish pizza for a while '
within minutes , his response had sparked outrage among internet users .
now , wallace has apologized for comment , which he dubbed ` offensive '
says he should have discussed singer 's ` remarkable talent ' - not weight .
clarkson , who gave birth to her daughter river last july , coincidentally discussed criticism about her weight with ellen degeneres same day .