At a given pixel dimension, the file size will vary a lot depending on whether the file is a TIFF, RAW (but what camera makes a file like this?!) or a jpeg - and in that case what the jpeg compression is set at.

I remember your previous posts - have you tried printing at less than 300dpi? If these posters are of a single image and you're not using a 155 megapixel camera, then you're interpolating up to that size anyway.