Regarding Procopius, we should also take into consideration that his numbers for the Callinicum are not trustworthy, as other sources seem to offer higher number for Romans, and of course if we take a look at his own account we can see he states both of armies were about 20000 and both have 5000 arabs with them, how many times such equity happens anyway? not to mention some of other problems in all of his works, including changing the date of Anoszads revolt in his History Of Wars which was published after the Persian wars.So we can presume he is quit capable of changing numbers and facts when it's suits him. as this is already evident in this Secret History when we compare it to his previous books.
Back to Dara, while Dara was built by Romans with the intention of using it as a base similar to Nisibis and while this was surely understood by Persians i am pretty sure at the time they didn't put much thought to it, until they faced the difficulties and defeats later on. you should also take into consideration two things, first Persians did not thought that they were going to face a pitched battle(it surely crossed their minds but it was not a strong possibly none less) and second exactly because of distance between Nisibis and Dara, they could always request more troops if they were needed(either for the siege or pitched battle).
I am not saying the Persian force was not larger, but the way is described is obviously false! i like to bring another point forward. as commander you always never count numbers of your fallen enemy first because you do not need to report the exact figure to your superiors, the exact figure that you should actually report are the numbers of your own dead which interesting enough Procopius never gives any! why is that? was it so low that he decided to not too? if so then why other historian which give detailed account like himself meaning Agathias and Ammianus give figure like 60-250 dead in some battles? wouldn't a low number of death being more glorifying, had A better reporting value? or was it that he didn't have access to that data? judging by his position this is very unreasonable.
My point is the description of these battles even on tactical part of is by no means without propaganda. he claims at start of his books that he is a witness and therefore much reliable but half of events which he describes he was not present at all and those that he was, he doesn't give the complete picture or falsifies the data.
Regardless he is the best source that we have but by no means, he is reliable when it comes to his favorite subjects before he wrote the secret history.