My teacher told me that to compare nucleophilicity, check the size of the attacking atom. Bigger the attacking atom more the nucleophilicity. But while reading Solomon, I found out that the order of nucleophilicity of halides in polar aprotic solvents is
Which is opposite to the order expected from my teacher's explanation. How can we explain this order. Can I conclude that my teacher's explanation is valid only in protic solvents, and in aprotic solvents, more the basic strength implies more nucleophilicity, or this is just an exception?