Sounds similar to when saving a file as a jpg (saved for web) that is lets say 12" x 12" @ 144 resolution, and you open that file back up, and it opens it up at 24"x24" @ 72 resolution. I think the pint being, the engine for (save for web) is set at only 72 ppi. People should not need to have a file (saved for web images) at 144 or higher than 72.
The same idea may apply within the Sep engine of that program. Let say for example, It has a set (res) assumed for each line screen that you tell it that you will be using. (I don't know if you assign the line screen inside the program or not), I don't use it. But it seems to make sense that it plays that kind of ball game. When you put in a 300 ppi file and tell it that it will be using a 60 lpi, (in theory) the suggested line screen is a max of twice that, being 120 res. It could be that when placed into another program, it's giving you the ppi equivalent) to the 120 rez and enlarges it to accommodate.
If this is the case, then try bringing in the file at 2 times the line screen you are going to use. (Do that even if it does not require the intended halftone info in the sep program) and see if it stays the same size. If it does, then thats it.
Don't really know.Just shoot'n in the dark here.
