Some difference in foreign country about MacOSX with Bootcamp

I always use Bootcamp in Mac. 
 A few days ago, there was a software problem in my computer, so I had to reset my laptop. Fortunately, I had done it several times. I knew how to install Bootcamp in Mac. But, there were some problems.

First, when I reseted my Mac, I needed to download MacOSX. But, I couldn’t. Because my AppleID was available in only Japan. I needed another ID in Canada to download.

Second, when I downloaded Windows10.iso(64bit), I noticed the difference between downloading from Mac and from Windows. The size and name were different.

・Windows : About 3.5GB
・Mac : About 4GB

I don’t know why they are different.
 I have already done with Mac one. But I still have worried about it.