Your only pulling in a 80mm pipe and the pilot bit probably cut a 120mm hole anyway, so it should be OK.
But then, halfway through the bore, you loose flow, you loads go through the roof and your package is stuck. It usually happens half way under the road or under a bunch of existing services where its not an easy job to just dig it up.
I am not sure if this has happened to you, but I know it has happened to me and usually on the last day of the job.