Images imported into Motion 5 always smaller

Hi, I'm trying to finish a closing credits scrawling animation. The size of my project is 3840 x 2160 px. The images with the credits that I'm plannng to use is 3840 x 18932 px. When I import it into Motion 5, the image's size is 1661 x 8189 px at 100% scale, instead of its native width, which is 3840px. I check the image properties in Media, the its Fixed Width tops at 1661px. I also check that Settings -> Project -> Large Stills is set to nothing.

My question is why it can go up to its native width of 3840 px?

Many thanks in advance.