I thought at first that the steam bot had a too high log consumption, but it turn out that they simply last the same time regardless if they absorbed a single or 10 log from their inventory, two bots doing the same job, one with 9 stone and 1 log and the other with 10 logs, both automatically refuelling, the one with 10 log consumed the 10 at once but did not last more than the one with only one log, he should either last 10 time longer or consume only a single log at each cycles.
Meanwhile, putting non wood object like stone on 9 of the 10 slots can help getting through this issue.