Gulp SASS, autoprefixer, minify and sourcemaps generate huge file

Question

I'm experiencing strange behaviour using this Gulp configuration to minify my SASS files generating sourcemaps.

'use strict';

var PATH = {
    SRC_DIR: 'src/',
    SRC: {
        IMG: 'img/',
        INDEX: 'index.html',
        JS: {
            DIR: 'js/',
            FILE: [
                'app.js',
                '**/*.js',
                'controller/**/*.js',
                'factory/**/*.js',
                'directive/**/*.js',
                'model/**/*.js'
            ]
        },
        SASS: 'sass/',
        TEMPLATE: 'template/'
    },
    DEST_DIR: 'dist/',
    DEST: {
        CSS: {
            DIR: 'css/',
            FILE: 'full.css'
        },
        IMG: 'img/',
        JS: {
            DIR: 'js/',
            FILE: 'full.js'
        },
        LIB: 'lib/',
        TEMPLATE: 'template/',
    }
};

var gulp = require('gulp'),
    autoprefixer = require('gulp-autoprefixer'),
    concat = require('gulp-concat'),
    minifyCss = require('gulp-minify-css'),
    rename = require('gulp-rename'),
    sass = require('gulp-ruby-sass'),
    sourcemaps = require('gulp-sourcemaps');

gulp.task('styles', function() {
    return sass(PATH.SRC_DIR + PATH.SRC.SASS, {
        sourcemap: true,
        style: 'expanded'
    })
    .pipe(autoprefixer({
        browsers: ['last 2 version', 'safari 5', 'ie 8', 'ie 9'],
        cascade: true
    }))
    .pipe(concat(PATH.DEST.CSS.FILE))
    .pipe(gulp.dest(PATH.DEST_DIR + PATH.DEST.CSS.DIR))
    .pipe(rename({suffix: '.min'}))
    .pipe(minifyCss())
    .pipe(sourcemaps.write())
    .pipe(gulp.dest(PATH.DEST_DIR + PATH.DEST.CSS.DIR));
});

gulp.task('watch', function() {
    gulp.watch(PATH.SRC_DIR + PATH.SRC.SASS + '**/*.scss', ['styles']);
});

gulp.task('default', ['watch', 'styles']);

The first time I run this script using gulp it'll generate a correct full.min.css and it waits for changes in the SASS directory.

When you change something inside SASS files it'll trigger the watch and re-run the styles task, which will build a full.min.css.

You see the changes in the front-end but this minified file becomes bigger and bigger at every rebuild.

If I omit the sourcemaps inside styles task it seems to work fine. The full.css file is always ok.

What am I missing here? Why the minified version keeps increasing its dimension?