using Instagram templates and dimensions

help please im using a lot of Instagram templates and changing the comp to the ratio i need,was wondering why all the Instagram templates come at dimensions too large for Instagram, as a result the designs are altered and cropped when i change the comp to the right dimensions to use.
is there a reason why the designers create the templates so large? when you change the comps to actual vertical dimensions say 864x1080 the tops bottoms and sides of the template is cropped out
would be great if the vertical templates where made in the correct dimensions
am i missing something? any help or advice would be very grateful.