I
would say the bacteria containing rhizobium is the bigger one and the one
without the bacteria is the smaller one. In the document, in section C, you observed the two. You can clearly see that rhizobium has bigger chunks than the one without it. The smaller one contains regular bacteria which makes it really small compared to the other
Here's some context I got from this website, it should help :)
"This
is because when the bacteria enters the cells and begins to reproduce, the
cells expand to make room for the growth of the Rhizobium colony and,
therefore, become larger than their neighboring cells that do not have
Rhizobium." (https://docslide.us/documents/biosem2516.html)
Hope this makes sense, good luck
-K12 student ;)