What do you mean by 'peasant' in the question. It's not clear to me what you're asking because the term is a loose one and not directly applicable to the Roman Empire.
This is a very difficult question to answer for several reasons.
The first reason is the timespan. I am assume you meant between 100 - 400 CE as BCE as the Roman Empire only began in 27 BCE. Even so the Empire changed greatly between these times with 100 CE being during Trajan's reign in which the Empire was largely stable (with limited expansion) and 400 CE being the reign of Honorius during which there was a constant struggle to maintain any semblance of an Empire.
The second reason this question is troubling is that modern archaeological evidence tends to contradict much of the classical authors accounts.
I will therefore try and answer this question not based on time, nor author but based upon a slightly different time period but utilising one of the most important papers on this topic. I will look at the post Hannabalic era, as there is a wealth of contemporary authors and also a great deal of scholarship into the period.
Following the defeat of Carthage, the popular perception is of villas run by slaves covering the countryside. The problem seemed so bad that the Gracchi brothers were campaigning to break up the estates to create land and jobs for the returning soldiers (1). Yet the evidence of this is not there, in 1981 Rathbone published what is in my eyes one of the most important papers on the subject of Roman slavery (2). He decides to look beyond the politicing of the authors of the Republican era and look purely at the situation using modern scientific methods. First and foremost he looks at the archaeological evidence, studying the remains of of the rural villas he notices something very significant. There is only a very limited amount of space dedicated to housing slaves, that is to mean there could not have been a huge number of slaves living permanently within the villa. This in turn led him to look at the economics of Roman slavery, essentially farming is a seasonal affair. At various points throughout the year there is a need to have a large amount of labour and at others it is not required, this means that for the majority of the year a slave staffed villa would largely be devoid of work but with all the expenses of food and upkeep still to be paid. Rathbone goes on to compare the economics of hiring seasonal labour with a limited slave workforce to that of a fully slave staffed villa and the latter is shown to be drastically more expensive.
This raises the question of which do we believe, the politicking of the authors or the physical evidence on the ground. Since Rathbone's study the perception of Roman slavery has shifted wildly, which is why when you read poorly edited sources like Wikipedia they tend to wildly vary between 10% and 40% slaves within a very small window of time. I am very much in the camp that believes that physical evidence trumps classical authors and believe that the numbers of slaves was vastly inflated by authors for a variety of reasons. By the Populares faction in order to gain traction with the urban plebs, essentially in the hope of breaking up the estates of the Optimates and releasing their financial control on Rome and by the wealthier authors merely to showcase their wealth. Keeping slaves was a luxury and as authors would write of their fabulous fish ponds (the most expensive hobby), writing about the numbers of slaves you had was an excellent way of showing wealth.
Sources
H. Boren. "The Gracchi." 1969
D.W Rathbone "The Development of Agriculture in the 'Ager Cosanus' during the Roman Republic: Problems of Evidence and Interpretation" 1981