Resize the left and right module positions

Please open \templates\[your template]\tpls\blocks\mainbody.php

In the mainbody.php the default line means:

"default" : [ "span6 offset3" , "span3 offset-9" , "span3" ],

"span6 offset3"  = main component
"span3 offset-9" = left side
"span3" = right side

// Layout configuration
  $layout_config = json_decode ('{
    "two_sidebars": {
      "default" : [ "span6 offset3" , "span3 offset-9"    , "span3"             ],
      "wide"    : [],
      "xtablet" : [],
      "tablet"  : [ "span12"        , "span6 spanfirst"   , "span6"             ]
    },
    "one_sidebar1": {
      "default" : [ "span9 pull-right"         , "span3"             ],
      "wide"    : [],
      "xtablet" : [ "span8 pull-right"         , "span4"             ],
      "tablet"  : [ "span12"        , "span12 spanfirst"  ]
    },
    "one_sidebar2": {
      "default" : [ "span9"         , "span3"             ],
      "wide"    : [],
      "xtablet" : [ "span8"         , "span4"             ],
      "tablet"  : [ "span12"        , "span12 spanfirst"  ]
    },
    "no_sidebar": {
      "default" : [ "span12" ]
    }
  }');

Here the configuration for the mainbody layout.

"default" : [ "span6 offset3" , "span3 offset-9" , "span3" ]

It mean, in default state (desktop window), the component will has span6 (width in bootstrap), sidebar-1 will be span3, span 2 will be also span3.
You can change it to "default" : [ "span8 offset2" , "span2 offset-10" , "span2"], so it will make two sidebar smaller and the main component will be wider