Would say BGT 2021 was slightly bigger, for some reasons. Firstly by the team the series had reached Gabba, India had a massively lower chance to win than SL had at any point during the 2018 SA tour, since the gulf between Ind and Aus in that match was much bigger than the gulf between SA and SL in 2018, Aus was unbeaten at Gabba for a long time, Ind was depleted with injuries so they would have been mentally weak as well. Secondly, Ind came back from losing the first match and getting bowled out for 36 only, so to win a series with a side that got weaker and weaker with each match was remarkable. Thirdly, four tests in BGT vs only two in SA tour. Fourthly, in the case of BGT 2021, it was truly a team effort, since if you individually look at the stats of the Indian players, they are not very impressive, Aus’s best batter and bowler were clearly ahead of India’s respective best players. In case of SA vs SL, Kusal Perera was clearly the biggest reason why SL won, with some other important contributors as well like V Fernando,