You're right, the overall % of curation rewards does not change. The author earns exactly 75% of the sum of vote values while all curators together get up to 25%. I was assuming (without explicitly saying, sorry) that the larger voter and the author are the same person/account/group. In that case, the author share is slightly increased by the curation snipers, but the curation share of author and large voter together can be reduced by more than this value.