compareWeightPercentile(siteImageWeight, percentileImageWeight, percentile) {
		if (siteImageWeight === undefined) {
			return;
		}

		let diff = (siteImageWeight - parseInt(percentileImageWeight, 10)) * 1000;

		if (diff > 0) {
			diff = chalk.red('+' + prettyBytes(diff));
		} else {
			diff *= -1;
			diff = chalk.green('-' + prettyBytes(diff));
			this.fasterThanAPercentile = true;
		}

		return diff + (' compared to sites in the ') + chalk.yellow(percentile.replace('p', '') + 'th') + ' percentile';
	}
Compare a supplied image weight with the weight in a supplied percentile.
@param siteImageWeight
@param percentileImageWeight
@param percentile
@returns {string} Summary message of comparisons to the percentile