I am sure I cannot lose weight to fit into the smaller pair.
Why would you buy a smaller pair if you don't think they'll fit?
My mom is always encouraging me to go down a size each time I buy clothes.
Having weight loss goals is a good thing, but its wasteful to buy smaller sized clothing if you haven't lost the weight to fit into them yet. You'll end up wearing something that'll leave you uncomfortable all day long. Or worse, you may end up splitting your pants and embarrass yourself.
If there is one thing I learned from my mom it's to wear the smallest possible size.
Pants that are too small for you are unhealthy as they cut off circulation. Find pants that fit you properly and don't listen to your mom in this area.