Saving struct arrays to disk with 'save'

I have a cell array of structs (spline bases created in the curve fitting toolbox) that takes up no more than half a Mb in RAM (workspace reads 424524 bytes). I want to save this to disk but using the standard Matlab 'save' command produces a file >5Gb (yes, gigabytes!) in size?!

Surely there is a bug with the save procedure for this datatype to be so inefficient? any workarounds?

Cheers