Looking at the number of individuals I would have regarded you as falling at the lowest point where you might start to have problems looking only at the number of individuals.
The file size does seem disproportionate to the number of individuals and I suspect this might be connected to the documents. I found when using FTMM that for every 30 images I linked my file grew by 1MB regardless of the size of the photograph I was attaching many of which were well below 10KB. I only link pictures of people, buildings , gravestones and the like, not census images (I have over 40,000) death certificates etc.
If it is size that is the problem then assuming that you have 4GB of memory on your computer to give FTM a fighting chance, if you want to keep using FTM then you need to look at the way you organise your file and your method of working. This may well centre in your case around the linking of documents rather than a simple reference to them. For instance I maintain a spreadsheet showing the image number I have given to census images rather than linking. The source attached to what I have typed into the program I am using is simply US Federal Census xxxx.