It could be because of a missing dimension (you mentioned that both the source and target module have the same dimensions) or it can be that one of the dimensions has a different level of granularity: for instance, perhaps time dimension (if applies) is in weekly buckets in the source and monthly buckets in the target?
Check the summary methods of the Line Items in the source module, too: set them to "none" and check the result of the data import.
This is what I can think of for now. Please, feel free to share some more details or a couple of screenshots and we will be able to find the root of the issue.
Re: Getting 4x the value of import in target module
When this has happened to me before, it's because I'm misaligned on the time dimension. For example, imported against year, but importing into month. From the 4x comment you made, I believe it would be a quarter vs. year mismatch.