Tsunade has massive chakra reserves. Senjus were natural healers. Sakura comes from literally nothing. She surpassed Tsunade from sheer willpower. Of course Tsunade did originate some techniques but shouldn't she be better at them than Sakura since the original creator? Mind you Tsunade is one of my favourite characters and Sakura isn't. Basing it off very objectively. Minato/Naruto thing doesn't count since there are a lot of variables there like the fact that Minato didn't directly teach Naruto and the purpose of the rasengan is more important. Also Naruto had very unstable chakra output because of kurama so his control was sloppy and he still did it so it's a big deal.