You are also forgetting that Dragon Quest XI launched on 3DS as well. A lot of the initial sales in Japan were from the 3DS. Extrapolating Switch Dragon Quest XI sales: It was at 4M as of November 2018 before the Switch version released and after the international release. By December 2019 it was at 5.5M, which is after the Switch version released. By September 2020, before the release of XI S on other consoles, it was at 6M. According to Famitsu the NS version sold about 550k copies before the "new price" version was released where it sold another 150k or so. Based on the numbers given the Switch probably shipped over 1M units. Especially when you factor in digital as well as international.
Anyway, Dragon Quest IX was on the DS. Dragon Quest X was on the Wii and 3DS. Dragon Quest XI was on the 3DS. Square Enix doesn't really care about power for the Dragon Quest series. It's honestly all about the Japanese audience.
