STEP 1. Is your .dwg file drafted in full-scale? If yes then draw a full-scale paper size in model space copy it as a reference size and then paste it to layout preset ANSI C. proceed to STEP 2 if NO, ensure that your MODEL SPACE drawing is drawn in full scale and uses your default units. Scale the entire drawing if needed so it represents a full size or scale.
STEP 2. In Paper Space, set your template, margins, scaling factor, etc. and using your created actual paper size in the model space, use that as a reference to ensure that you are indeed using a full-size paper in you layout or paper space, then you can create a viewport via MVIEW command or insert view under layout, anyhow if things look so small or so big in your viewport, please ensure that your scale factor is working properly with layout space. Apply scale factor when necessary.
This one of the reasons I think? If you still have an issue we are glad to hear from you!