Sounds like you're lucky the amount is wrong cause you aren't happy and you should be able to return it under these circumstances and get a refund.
Which fragrance was it? I find that when something doesn't smell right to me I'm usually correct even for pretty small differences between one bottle and another, but not 100% of the time. It's best if you can compare two bottles side by side to form a conclusion about how similar the contents are.
Clearly getting the top off one of those bottles isn't as easy as one two three though.