800F Freelance project size
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Check the size of your PRO file. In earlier versions there was actually a limit. I think that limit was based on the 2 GB of RAM an application can consume in windows. I'm not sure how it is nowadays.
However, your PRO file is growing over time. The most effective way to shrink it back to its minimum size is to do an export to csv and import back again. But that requires loading whole station and therefore a stop of production.