the sweater is too expensive,please show me a cheap one.第二句:

the sweater is too expensive,please show me a cheap one.
太贵了,比它便宜点不一定够,所以要便宜的cheap one,而不是cheaper one.
if you don't like the red coat,how about the green one?
---Ok,but do you have a bigger size in green?this one's a bit small for me.
这个是跟正说着的红色相比,而比它大的当然有很多码数,而要的只是比红色的大一点就可以了,所以用bigger.

第一句没有比较的意思，expensive和cheap是两个相反意思，第二句是同一颜色的型号比较！

可以细细体味，毛衣指代问题，1随便拿一种便宜的，2就是手中的那件只是型号大些。