The choice between the most appropriate format for representing and exchanging data is of great importance as it impacts on the performance of the various applications that use them. Currently XML and JSON are the most commonly used formats for this purpose. In this article, we will describe the comparison of these data formats in terms of the size of the file to be exchanged. An evaluation is made with the files in their natural state, i.e. without conversion, and compressed, using a public compressor. The data conversion time and the size of the compressed file were compared. The obtained results were the same for the two when compressed 1 and 10 registers, however for number of registers greater than 1000 the XML proves to be smaller than the JSON.