User blog comment:Kaido King of the Beasts/Chapter 879: Mirror Battle in Mirro-World/@comment-24850639-20170922150027/@comment-24769684-20170924001122

Using the Grand Fleet to rescue Sanji would have been an even worse idea. Sneaking around would have been absolutely impossible, and while Big Mom did know about the SRT's location most of the time, their small numbers resulted in them not being swarmed with high-level opponents. The Grand Fleet currently numbers around 5000, while Big Mom has tens of thousands of Chess Soldiers alone. Most of the Grand Fleet are just mooks, and I don't see most of the captains being able to hang with the Sweet Commanders and definitely not Big Mom. Plus, the SRT's small size also allowed them to run away from Big Mom. No one could have run away in a struggle of thousands of fighters. They also only have 68 confirmed ships, which the Big Mom Pirates almost certainly outnumber. We saw what happened when the team tried their hand at an all-out battle at the end of the wedding ceremony, with a Supernova crew and a powerful miltary nation at their side, and how they were quickly decimated. A full-out territory war would have less of a chance of success than the SRT sneaking operation unless Luffy didn't care about his forces being decimated, because the Grand Fleet does not have the numbers or strength to fight the BM Pirates evenly.

Bringing the Whole Crew to Whole Cake might have worked, but doubling their numbers would only make them easier to spot and pounce on. Remember that at the beginning, Luffy wanted to do this on his own; the others joining him was more or less an outright effort to give them screentime to compensate for their lack of it on Dressrosa. This was supposed to be much more of a side mission than it actually ended up being, and Luffy wanted to ensure his crew laid groundwork in Wano which is considerably more important.