For some years now I've used Li-Ion lightweight small-size bike batteries with great success, especially in very cold weather. Only trouble is, the Li-Ion battery tender or Intelligent Charger is hard to find and you need to have one 'cos you're Optimate - or similar - for regular old-style batteries won't do the job.
Also some Li-Ion batteries come with a set of foam packing slabs to fill out the space normally taken up by an old-fashioned battery.
And there's a simple solution to the slowing-down effect of very cold weather on a Li-Ion bike battery : - "As a result of their unique chemistry, lithium batteries are known to slow down in cold weather, and will behave as though they are dying (they actually don’t lose power, they just discharge it more slowly.) There is an easy solution for this – simply “warming up” the battery by running the headlights for 30-60 seconds before pressing the starter button."
That works perfectly and then the starter spins a freezing bike motor like gangbusters.
I also follow the maker's tip to unhook the battery neg pole when leaving the bike unused for a month or more. You can buy a simple screw-down switch to mount unobtrusively near the battery so that all you do to disconnect or re-connect is lift the seat and twist the knob.