The ebook consists of several parts
1. A fixed amount of overhead for the viewer code. About 480K in version 4.01, and about 410K in version 4.22
2. In 4.22 only, the toolbar and language data, should be <10K (compressed)
3. Your splash screen if present, compressed
4. The settings for the ebook, like the title, passwords etc., This should not be too much (a few K probably)
5. The content. Compressed. HTML files compress very well, GIF and JPEGs not very well. HTML code might compress to around 25% of the original size.
Add these up, and you probably get an idea of why your ebook seems so large, it's the fixed overheads
500K is probably (I'm guessing for your ebook as I don't know the exact figures), 485K or so of overhead, and 15K of compressed content (the 54K compressed)
If you have say 1000K of content, and it compresses, you'll probably find with Activ, assuming similar type of compression ratio, it's 485K + 250K (25% of 1000K) = 735K ebook
Couple of differences from most of Activ's competitors
(I am going to try and avoid naming specific ones, as I don't spend my days tracking each one individually, so forgive me for using broad generalizations)
1. I suspect our overhead might be larger than some competitors. The reason for this is they compress their viewer code. We don't. There is a good reason why we don't -- if you compress the viewer code it causes compatibility problems with some anti-virus products. Personally, I think an extra 100K or so on downloads in the overhead is a small price to pay to know your ebook will work regardless of which anti-virus product your customers have.
2. We compress the content, and pretty well I think. Some of our competitors don't. What this means is if you have a lot of content in your ebook, Activ will usually make a smaller ebook
Finally go look at the Q+A on ebook size in the reference section of the Help. This has some tips on helping keep the size as small as possible.