Cryptography ensures data privacy by transforming data into unreadable formats that only authorized individuals can decrypt. With the increase in electronically stored and transmitted data, enhanced methods for data protection are required. DNA cryptography, leveraging the genetic structure of DNA, provides a promising approach for secure communication and data storage. This paper introduces a novel DNA-based cryptographic method employing a DNA one-time pad (OTP) combined with modified run-length encoding to reduce ciphertext size. Unlike traditional cryptography, which often results in a larger ciphertext than plaintext, our proposed method demonstrates a significant reduction in ciphertext size. Experimental results reveal that for input text files of 1MB, 2MB, 3MB, 5MB, and 10MB, the ciphertext sizes were reduced by up to 20KB, 40KB, 60KB, 100KB, and 200KB, respectively. This reduction not only enhances storage efficiency but also minimizes transmission costs, marking a substantial advancement over existing DNA and classical cryptography methods. Future work will explore the application of this technique for encrypting biological data and incorporating DNA barcoding for improved data authentication and reliability.