'... they are easily deployed in large numbers due to their small size, light weight and the fact that one trainer can work with many animals...' We use 'due to' to state the reason(s) for something. However, note that we must use a noun after 'due to'. There are three reasons given here as to why rats are easily deployed in large numbers: (1) their small size, (2) their light weight, and (3) one trainer can work with many animals. Bearing this rule in mind, we can keep (1) and (2) because they're both noun phrases. However, for (3), it would be incorrect to say 'due to one trainer can work with many animals'. We have to turn it into a noun clause by adding 'the fact that'. When the reason is not a fact, use other words that suit the circumstances, e.g. assumption (He stopped calling her due to his assumption that she was not interested in him. He should find out for sure before giving up), situation (Due to the unpredictable situation that we are in, I suggest we do not proceed with our plans), and so on.