Sequence TimeLine Sizes Keeps Changing?

Hi, every time i put one of the videos into my sequence that  i created which has a custom size 2560 x 1440 for the image that ive imported i set the sequence to 2560 x 1440 the first 4 images  or so it fits right into the size sequence then when import more images that are the same size i have to keep resizing them , its not making sense?