You’re confused. The “big sperm” is an advantage for the shrimp.
Here’s an analogy: A long distance runner wants to improve his stamina. A sprinter wants to increase speed.
They both have different strategies to meet different goals.
Is one of them somehow fundamentally “better” ?
If your environment is man-eating animals who can’t run faster than 5 mph, the sprinter is fundamentally better. If your environment is sparse vegetation where you have to travel miles a day to get enough food to eat, the long-distance runner is fundamentally better.
Or you could just accept that whatever happens to be seen, it must have been what was “better”. And therefore, we could just look at whatever happened in the past, and take whatever evidence we find and make up a story about how it is better.
It’s like how each day, the papers “explain” why the stock market went up or down. They always have a perfectly good-sounding reason, and you wonder why these people didn’t use their knowledge of what the market would do to get a better job than writing stories about the market — until you realise that they can find an “explanation” no matter what the market does.
I’m sure these researchers could have convinced us that small sperm was a great advantage for the shrimp as well, if that was what they found.