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