Americans have been migrating south and west in larger number since World War II, and the pattern still prevails. Three sun-belt states -- Florida, Texas and California -- together had nearly 10 million more people in 1980 than a decade earlier. Among large cities, San Diego moved from 14th to 8th and San Antonio from 15th to 10th -- with Cleveland and Washington. D. C. dropping out of the top ten. Not all that shift can be attributed to the movement out of the snow belt, census officials say, Nonstop waves of immigrants played a role, too -- and so did bigger crops of babies as yesterday's 'baby boom' generation reached its child bearing years.