It's not a bar near closing time. Sure females of all species select the most suitable mate, no matter what that may entail, but like the movie quote, if there is only one scrawny male around, he's like "so you are saying there is a chance?". And larger fish do produce more eggs, by virtue of their size, but they aren't rare because they are different, they are the rare because of statistics, picture it like the chances of a particular person winning the Lotto vs someone winning the Lotto, one is extremely not likely, the other is almost a certainty.