I think it works like this:
The avatar dimensions only get stored in the database if the avatar is uploaded though My Home.
When the avatars are displayed, if the dimensions are present, they're used for the width and height attributes. Otherwise, the default dimensions from the theme settings are used for the width and height attributes.
The import script just stuffs the avatar URLs into the database. It doesn't set the dimensions.
You can use this query to see what's in the database:
SELECT U_Username,U_Picture,U_PicWidth,U_PicHeight
FROM w3t_Users
WHERE (U_Picture is not null and U_Picture <> '' and U_Picture <> 'http://')
OR (U_PicWidth is not null and U_PicWidth <> '')
OR (U_PicHeight is not null and U_PicHeight <> '')
