The lightest batteries with the highest available amperage I found are Hawker energy Cyclon series batteries.
Just compare the max amperage of comparable batteries to them, the difference is huge.
Also, since starting power is not only depending on the battery, but also on voltage losses, consider increasing the wire diameter of the leads going to the starter motor, but always use
highly flexible wire. In some cases I used heavy gauge loudspeaker wire, when nothing else was availble. Works great.
In some cases I have found cheap starter relays can create useless voltage drop, too. I tend to use
the next bigger model for what I want to do, e.g. a relay for a Lycoming starter, as sold by Aircraft Spruce.
