Adjective before classifier
大 or 小 before a classifier describes the size or amount of it
Example:
一堆 = a pile of; 一大堆 = a large pile of;  一小堆 = a small pile of
堆 is not a measurement unit but a description of a form. The adjective 大 or 小 roughly describes the object's size in this form
In the case of 一大亩, since 亩 is a measurement unit, there is no difference in size between 一亩田 and 一亩地; 這一亩 and 那一亩. The adjective '大' (big) is, therefore, a subjective description for emphasis (the speaker considers 一亩 is a big area)

一 (number)

大 (size/ amount)

亩 (classifier; measurement unit)

一亩田 = one mu of field (the speaker is stating the actual size of the field, which is 667 square meters)

一大亩田 = one big mu of field (the speaker consider 667 square meters is a big area)


Another classifier for 田 is 片, which is not a measurement unit:
一片田 = a field
一大片田 = a large field
(大 or 小 roughly describes the size of the classifier 片)