The substrate is perfectly fine as long as it is kept clean and there is nothing to be ingested. Frogs that are kept on non-burrowing substrates also grow faster, as they are never hiding where they can't see food. Your frog just needs a water bowl it can more easily get into. A lot of these frogs simply do not have the genetics to get huge. Phillipe de Vosjoli has pictures of multiple-years-old frogs that are barely larger than a quarter. I honestly cannot tell what gender this frog is, size wants me to say female, but it does have some male traits.