A 150/70 will physically fit inside the swingarm but is far too wide for the stock 2.75" rim. If you try to fit one, the rim will pull the beads in significantly, making the tyre egg-shaped in cross-section. It will make the handling terrible, the contact patch tiny and square-off so quickly you'll end up throwing it away after a few hundred miles.

A 140/80 is the biggest a 2.75" rim will safely take, and even then in my experience it still gets slightly pulled out of shape and negatively affects the road handling and squares-off quicker than a 130 of the same brand and speed rating. As Pete mentions, narrower is better off road in the vast majority of conditions. I also found with a 140/80 the traction off-road (on grass, mud and loose gravel) slightly worse than a 130. IMHO a 120 is the ideal for off-piste in all but sand, where a 140 has marginal advantages.

Unless you are desperate to aspire to the more macho look of a wider tyre, I'd personally (IMHO) stick to a 130/80. It is the best compromise. Perhaps not surprising then that it's the OE tyre size!

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