Crecer y disminuir DIV con javascript

function AumnetarTamanio(pAumentar){

var tabla = document.getElementById("divDinamic");
var anchoTabla = tabla.offsetWidth;
var altoTabla = tabla.offsetHeight;

 altoTabla = altoTabla + pAumentar + "px"  
 document.getElementById("divDinamic")
 .style.height =  altoTabla ;

// .style.height="400px";
//alert("NewHeigth " + altoTabla) ;

 }


 function DisminuirTamanio(pDisminuir){

 var tabla = document.getElementById("divDinamic");
var anchoTabla = tabla.offsetWidth;
var altoTabla = tabla.offsetHeight;

 altoTabla = altoTabla - pDisminuir; //+ "px" ;
 altoTabla =  altoTabla + "px"; 

 document.getElementById("divDinamic")
 .style.height =  altoTabla ;

 }