why can't asp.net find the appropriate apicontroller when input is large?

I have a function that saves a list of my entities at once.

public void Save(IEnumerable<SubjectData> subjectDatas)
{
    var request = _requestFactory.CreateRequest("api/subjectData", Method.POST, AccessToken.AccessToken, new List<SubjectData>(subjectDatas));
    var response = Client.Execute(request);
    _responseDeserializer.Deserialize<SubjectData>(response);
}

That's calling a wep API function:

// POST api/<controller>
public void Post([FromBody]List<SubjectData> values)
{
   _subjectDataService.Save(values, User.Identity.Name);
}

when I subjectDatas is a list of about 30, this works fine. However, when subjectDatas is very large (in my test case, over 96000), I get an unexpected error. The response has StatusCode NotFound. what's going on? Why can it suddenly not find the right controller?