Could the expressions “than” and “seem” be used in similes?

Based on what I learned from my high school teacher of English, a simile is a figure of speech in which one thing is likened to another dissimilar thing by the use of such expressions as like, as, and as if. My college instructor, however, commented that even the expression than could also be used in a simile (e.g. “The astronomer’s head is much bigger than the Jupiter”). What could be the instructor’s basis for having made such a remark? Another thing: Could the expression seem be used, too, in a simile (e.g. “At night, the mountain on which numerous residential houses were built seems to be a giant heap of multicolored gemstones”)?