Bookeen Cybook Opus large epubs

Hello all!

So I still have the Bookeen Cybook Opus ereader and was thinkijng to give it away for someone who would enjoy it but it seems that large EPUB files cayse the reader to crash and reset. Is there a trick to make these work with Calibre? Smaller EPUB files work just fine, I think the limit is 1.5 Mb or so.

Thanks for any tips!