Those who ate alone did not have as large a drop in their cognitive processing as those who ate in the restaurant.

这是“as...as...比较结构”，比较的是两个宾语的程度大小。第一个 as 是程度副词，第二个 as 是比较连词（也叫比较代词）。

not as...as... ：前者不如后者；前者没有后者程度大。

1. have a large drop in their cognitive processing  他们在认知处理能力方面大幅下降

2. 变成比较结构时，因比较项 large 被as 特别强调，故放在 as 之后，冠词之前：have as large a drop in their cognitive processing as...