Some creatures' valid level ranges vary depending on character level, and therefore their soul size changes depending on what your level is. Grand - Mammoth, High-level Dragur 'boss types' (ie, Dragon Priest).Greater - Giant, Storm Atronach, Low-level Draugr 'boss types' (ie, Deathlord at low levels).Common - Snow Bear, Cave Bear, Frost Atronach, Frost Troll, Draugr Scourge.Lesser - Restless Draugr, Sabre Cat, Bear, Troll, Falmer and Falmer Skulker.
Here's a quick chart of the most common types (or at least the most common types of enemy I tended to encounter on my playthrough), but there's a full listing over on the UESP should you be curious about a particular type of enemy: Enemy souls are determined by their level, but without the console or an encyclopedic knowledge of enemy statistics, this information is not particularly useful.
