By default, pictures in a Grid Collage are positioned such that they are just bigger than the frame in which they are put. Pictures are scaled uniformly (maintain the image's aspect ratio) so that both of their dimensions (width and height) will be equal to or larger than the corresponding dimension of the frame. You can also opt to fit a picture inside its frame so that it fits completely within the frame. Note that in this case, some blank space is left in the frame. This option is useful for creating contact sheets.
Go to Settings tab in the left side of the application.
In the Layout section, select Inside for Fit Pictures option.