People just have different opinions.. I use 16oz cleto's and they are fine.. Our trainer isn't to fussed what size we use... Most people recommend using at least 16oz to protect your opponent, help condition your arms etc.. But I guess having really big puffy gloves and learning to defend with them may be a little different to defending with 12oz, but I don't think it would be a huge difference.. If your using good form with your defence, stepping down gloves shouldn't be to much difference... I wouldn't use really light gloves to spar just for that reason...
I think picking between 16 or 18 is just a matter or preference... Plus certain gloves are different as well as far as padding goes.. My Cleto's are twice as hard as some of the other guys gloves in the gym who have 14oz... So maybe your trainers comments were as much specific to your gloves as to the size of them.