I think it depends on the software (and its prefs) that created the .jpg.
Open it in Graphic Converter. Check the GC prefs. You can choose the size, etc. of previews, icons, etc. That'll probably let you get the correct preview that you wanted associated with the .jpg.
PS: Although you can buy it - there is an "Apple" edition of GC that came with Panther (and Jag, too, if I remember correctly).