My guess is they infiltrated the pager supply putting the bombs in all pagers and triggered them with Hezbollah's own encoded message system so only the guilty parties would be effected.
Looked like 5-10g (maybe up to 20g?) of explosive, NOT battery. I think you could fit the whole package inside an AA battery, along with an AAAA battery, so you could do something crazy there, or just replace a rechargeable battery pack with something of smaller battery capacity containing the explosive, some electronics, etc. Or just use spare volume inside the case and hope no one does gross physical inspection.
(This is an assumption, I'm neither a doctor nor a terrorist).