When you scan, look for any zoom levels. Things should be scanned at 100%, they may be set to something like 125%?? or larger? Also look for caclulated scanned dimensions. Let's say your pages are 4 x 6. Do they say 4 x 6 or 5 x 8?? or something also larger than you want?
Oh, if you cannot see what I'm referring to above, one other thing to look for is that many scanning software has different "views" depending on the user's interest or capabilities. So, for example, it may say something like "Simple," "Moderate," or "Experienced" implying that the user can select which mode they wish to scan in. As you go up in levels, the more options will be displayed.
Also keep in mind that Acrobat cannot scan. Rather it connects to either your scanning software via TWAIN if you are on a PC or Apple's Image Capture if you're on a Mac. Either way, the controls you are looking at are those, not Acrobat's.