大页面里的js:

<script>
function jmp(){
var w = document.body.clientWidth;
if( w > window.screen.width){
w = window.screen.width;
};
if( w < 1003){
window.location.href="2.html"
};
}
window.onresize = jmp;
jmp();
</script>

小页面里的js:

<script>
function jmp(){
var w = document.body.clientWidth;
if( w > window.screen.width){
w = window.screen.width;
};
if( w > 1003){
window.location.href="1.html"
};
}
window.onresize = jmp;
jmp();
</script>