I think that you should be able to find different approaches to this particular question. There is a physical challenge of a classroom with a large number of students. I think that the physical element of a class with forty or fifty students is daunting. There is only so much time in which a teacher can provide general instruction as well as differentiate content in accordance to student learning needs. A large class makes this personalized approach to teaching and learning a fundamental challenge. As more students increase the classroom environment, there is a larger chance that this makes a more heterogeneous setting where student needs and instructional demands are more diversified, making instruction more of a challenge. This is another reason why larger classes could impact the learning environment. I think that classroom management techniques are more in demand with a larger group configuration. It is a bit easier to control a smaller group of students. If one has to instruct a class of 45 students, there is a more pressing need and demand for strong and tested classroom management techniques, an area that is challenging for even the best of teachers. I think that this becomes a fundamental challenge in a large classroom settings.
And to be even more realistic: large class sizes are detrimental to all parties involved--there, I said it. However, the term "large" is relatively vague, as a "large" class to one in a lecture hall may mean upwards of 100...while "large" to a sixth grade science class may mean 30... So many variables would affect this. What is the grade level/subject being taught? What is the need gradation--in that, how many students with special needs? How many non-native speakers? How many students are concurrently enrolled in an ESL program? How many students require "accommodations"? How many aides would be in the room to assist such needs? Etc.
As per instruction, well, yes--any teacher worth his/her salt should be able to address the needs of several levels during one instructional period. However, the larger the class becomes, the more difficult that becomes on any one teacher. Differentiated instruction and tiered learning initiatives are something that we all do, or strive towards, but the more administrations pile students into one given learning setting, the more difficult it becomes on all involved. And ultimately, the less likely that all needs will be addressed.