Yes indeed it is true, its also true that when a guy see a lady he fancies he will raise his eyebrows on first contact with her - this can be hard to spot as it happens very quick but is a subconscious reflex action...
yeah the reason for it is that the brain likes what it sees so the pupils enlarge to let in more of what the brain likes apparently.
My uncle came to visit when I was a teenager, I was in my bedroom with my boyfriend having taken some drugs, he told my mum we were getting naughty cause my pupils were so big.