As a TWICE fan, this may come off as biased but i do not think Spring Day is bigger than Cheer Up.
Spring Day is called the biggest national hit cause of its longevity but they're was never any craze around it.
You dont have grandmas dancing to it like they did for Dynamite or Cheer Up.
I think Cheer Up, Love Scenario, Dynamite and even to a certain extend D4 are all bigger than Spring Day because the song marked people and was crazy viral. For me it seems like Spring Day is just a hit. There's nothing about it that stands out.
Spring Day is like a minor hit that lasts forever in the top 40 wherehas other songs i mentionned topped the charts for months and are known by far more people than songs who stay in top 40.
Im ready to bet money that if you were to survey people on the streets, they'd remember Cheer Up, LS, D4 and Dynamite more than SD. Even if SD charted 3x times longer than thoses songs, im sure people would recognize a "Shy Shy Shy" or a "Hit you with that D4" more than SD
Love Scenario and CU are still mentionned in variety shows frequently.