As noted, the first version does appear counterintuitive; if an action is prohibited because it will definitely bring about a melakhah (psik reisha), larger chairs are even more likely to cause furrows than smaller ones. Rabbeinu Dovid (Pesachim 25b) explains that in the case of the larger chairs, it is impossible to transport them without dragging them; thus, a furrow caused by that dragging can be attributed to a completely unintentional action (davar sh’eino mitkavein). However, smaller chairs can be transported without dragging them; thus, dragging them, and consequentially creating a furrow, has the stain of negligence.
As noted by the Pri Moshe (Shabbat, 4:4), at issue here may be the question of why it is that “psik reisha” creates responsibility even when the result is unintended. There appear to exist at least two possibilities: a) as the result will definitely occur, it cannot legitimately be considered unintended; b) the result becomes an extension of the original action. If the first explanation is dominant, it seems logical that larger chairs, that will even more definitely create furrows, are without question prohibited. If, however, one uses the second interpretation, it might be argued that the secondary result only becomes a part of the initial action when the actor had the possibility of doing that action without it; thus, the consequence is considered deliberate. If, however, there was no way of performing the action without the consequence, that consequence remains unintended.
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