Question
Basically she needs to explain why if she planted the same plant in two different size pots, what is it that makes them grow different sizes? She has an idea but needs more detail. I looked all over the internet for the reasoning and it's just not a question most people want or need to know.
Thank you very much.
Crissy
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Followup To

Question -
I have a friend who is trying to do a project for school and is trying to find out the difference in the way plants grow in both small nd large pots. Like if you use the same amount of seed, yet they grow different sizes.
Thank you very much.

Answer -
Hi Crissy.  I'm trying to do a little mind-reading here to figure out what your question is.  Are you asking me to predict what I think your friend will find in this little experiment?  Or what kind of plants to recommend for this research?

A fast-growing plant, such as Radishes, which you can eat when you're done, would be my choice.  Radishes are ready to pick at about 8 weeks.  People like to grow them next to Carrots so that they can gauge how much longer the Carrots need to be in the ground after the Radishes are ready.  Carrots take a few weeks longer, give or take a few days, depending on the Carrot.  Radishes are easy and fast.

Do you have any question that I have not answered?

Answer
OK.  I think I have a better idea now.  Thanks for clearing that up for me.

I hope we can agree that the bigger pot will produce a healthier, larger plant, yes?

The smaller pot is providing an environment similar to the situation you would have if you were planting a lot of plants close together outside, or more than one plant in the big pot.

Plants don't like to compete for nutrients.  They need water, soil, air and minerals.  If they have to share it with weeds, or with another plant, they will get less, and that loss will show up as smaller growth and smaller flowers.

A smaller pot very simply does not hold the same amount of good stuff that the bigger pot holds.

What people are beginning to realize, in fact, is that the microbes in the soil - the tiny microscopic bacteria and fungi that live in the dirt outside - are part of the growing that needs to be done.  They make a continuous supply of minerals and nutrients for the plant.  And the plant's roots are in charge of obtaining those nutrients and sending them to the stems and leaves.  A smaller pot has fewer bacteria.  It has less of everything.

Does this explain the situation?  I can get more technical or go into more depth about any or all of the above.

In fact, let me add one more detail:  Plants need PHOSPHOROUS to grow strong roots.  If one plant has less Phosphorous, it will be just like putting the plant in a smaller container, or competing with other plants in the ground.  The plant will not be able to grow healthy roots; smaller roots cannot do the good work of a large root system.

So, how do you like them apples?