As listed in the comments, the issue is the DPI. When using the A4 template the default DPI is 300. Your original images were created at a different DPI. A typical DPI is 72 (which is also the GIMP default), whereas for print it is generally 300 (which is what will be selected if you use the A4 template in GIMP).
As @Paolo Gibellini mentioned in the comments, the formula for DPI is
SIZE_IN_INCHES * DPI = SIZE_IN_PIXELS
(In your case, you would need to convert from CM to inches first). Using the dimensions that you provided (160 pixels for 5 CM), the DPI can be determined by solving the below:
(5 * 0.39370) * DPI = 160
Which comes out to
DPI = 81. Setting your DPI do that should make your images import at the correct size.
Alternatively, you could also keep the 300 DPI (which might be desirable if the result is intended for printing) and scaling the images using the Scale Tool in GIMP instead.