SharePoint 2013 applies global styles to the inputs that affect the appearance of the RadButtons.For the time being the styles that SharePoints sets to the inputs can be overriden:

ASPX:
<telerik:RadButton ID="RadButton1" runat="server" Text="RadButton"><Icon PrimaryIconCssClass="rbAdd" /></telerik:RadButton>

CSS:
<style type="text/css">
    .RadButton.RadButton_Default.rbSkinnedButton input {
        padding: 0px 10px 0px 25px;
        border: none;
        margin-left: 0px;
    }
</style>