Simple surface model becomes a huge file

I am working on the design of a cowling for an amphibiousairplane. I have found that boundary surfaces are the most stableand controllable form of loft. This part is composed of a sweep,and two very simple surface lofts, which are knit into solidscombined, fillets added, shelled and finally mirrored.

The file begins at about 7MB, after further work it doubles, anddoubles again, and has been seen as large as 330MB. Some miserabledata structure is growing. What can I do to get it down to areasonable size?

SolidworksSurfacing