Photographic characters and file sizes

Hi

We created a project with 2 slides and 2 characters per slide and 15 layers.per slide.  On each layer the character state changes.

originally this was created with the atsumi character  as both the person who created on hasn't got a character pack and me to chane the character to ones in the character pack i have.

originally with the atsumi character the file was 35mb - way too big for our courses but I can break it down somehow.  When required i right clicked on atsumi and choose change character.  I saved the file with the new characters and the file size is now 68mb!  as this was just a straightforward   replace why has this happened?

Would the file size reduce with illustrated characters?  can you replace a photographic  character with an illustrated one or vice versa without going through each layer and state?  I noticed when I replaced the photographic character it occasionally changed the state to a comparable pose i.e. angry atsumi to angry Brian.

Thanks

Michelle