Hello Noah. I had set the motion project length longer already.
I found something interesting though: when I check the media through the inspector, I find a 5 fps cadence. This corresponds to the length of the photoshop images I set in FCP (this way I just place the images one after the other one to create my animation without having to redimension the images inside the timeline, and it does 5 x 5 = 25 images per seconds when I render). It looks like Motion finds in the rendered video an information that defines the fps at 5 fps while the video is actually at 25 fps (HDV1080i50). Now, I think the source of the problem is there, what do you think about this?