Dear rmacfadyen,
Did you limit the value of upload size? You can remove this limitation and set this value to larger and try it again:
<CuteWebUI:Uploader runat="server" ID="Uploader1" InsertText="Upload Multiple Files (Max 10M)"
MultipleFilesUpload="true" OnFileUploaded="Uploader_FileUploaded">
<ValidateOption MaxSizeKB="10240" />
</CuteWebUI:Uploader>
Thank you for asking