Some information may not be as meaningful to the adcoms, is my guess. Try focusing on the most impactful and meaningful components of your project rather than on every last detail. I'm applying MSTP so I had to basically write the same essay but the SRE essay allowed 10000 characters and I managed to discuss more than 3 research experiences, so you should be able to do it for one in under 3500 characters, provided you limit the scope of your discussion of the details. How detailed are you being about the hypothesis, methods, results and interpretation?
Another strategy is to work the meaning of the experience into the introduction and other sentences as you explain the experience, rather than having a separate section about that. IMO, it makes reading the research of various fields of many individuals a lot easier on the reader when there's a clear direction drawn from the experiences and discoveries as they read, rather than reading a lot of text detailing the specifics of the experiment without clear motivation, learning, or personal change until thereafter.