I also agree that bedgraph is the format that would be more helpful for your interests. you can define start and end chromosome positions, plus a value that usually represents coverage, but that you can use it to represent the methylation b values instead. you can still gzip the file, and IGV will still be able to deal with it. a similar approach (reducing file size to improve data storage and make data access faster) would be to transform that bedgraph file to bigwig format using UCSC's bedGraphToBigWig (find it here), which is an indexed binary format that works great on IGV. plus you can apply colors to this track on IGV depending on thresholds, which is useful to detect high and low peaks for instance.