They’d love Sam all the same. After seeing how him and Dean are with the others that were captured, Walt realized that size really didn’t matter when it came to his adopted sons… because after everything else, he’s adopted Dean as surely as he adopted Sam. They just don’t quite realize it.
There would be a lot of shock, and it might be bittersweet to see Sam ‘regular’ sized after all those years raising him up as one of their own. And Sam would be sad to realize he could never go home again, because he’d never fit. But he’d be where he belonged, with his brother, and he’d never forget them.