I recently took my little mezzo from London to Melbourne and back. I spent some time playing with how to pack it. I took the handlebars off, and of course then the front forks were loose, so I pulled them out. The whole thing squashed down more, so now it would fit in a normal, largish suitcase.
But then I looked at it. It was a chore to dismantle and reassemble, and realigning the handle bar so it all folded nicely was also a chore. A lightweight suitcase is more than 3kg, probably more like 4 or 5, and with that you'd have to reinforce with something, as lightweight cases don't offer much protection from crushing. Economy luggage allowance is only 20-23kg from the UK, so that's almost my entire luggage allowance gone. Also, when the bike is folded, the whole drive train is protected within the fold. Compacted down, it lost all that, so it might even be more vunerable.
This, combined with the fact that my accountant told me it was completely depreciated (it's a 4 year old bike, that belongs to my company), I just folded it, strapped it together with velcro to make sure it stays folded. I put it into a soft bag I already had.
I packed my clothes in compression bags, to protect them from grease, and to make them a suitable shape to protect the bike. Zipped it up, wrapped it in two more luggage straps and checked it in. Arrived at the other end in good shape, and nothing to arouse suspicion with customs and quarantine into Australia**.
Had a lovely time with it, was able to get about with a lot of freedom, and I did not put on 5kg in 2 weeks like I normally do. And had some lovely pootles around the eastern suburbs
before my parents woke up.
If I travel again, I'll do the same.
** Australia is rightly paranoid about pathogens coming into the country. They specifically ask about sporting equipment that has dirt on it. I believe they will hold any such items for cleaning and charge you for it, plus a fine if you haven't declared it. Though I'd given my cycle a good scrub before packing it, I was just as happy not to have a zealous customs officer inspecting it.