How old is Nacho? This is almost exactly what happened to us with Floris, but her little lump was by her ear. The vet felt it would be difficult to operate if it was left because if it grew there would be little skin around the ear to cover the wound. Plus it was in a tricky location, close to her eyes we didn't want to risk her losing her eyesight or it affecting her brain due the lump growing. It was more out of potential harm it could do because of its location that we decided to go for the operation while the lump was still small. It was really successful, Flo made it through perfectly and has never have any further problems or lumps.
It turned out it was just a fatty deposit I think they are called lipoma? So it wasn't at all harmful, except for its location. It is a really hard decision to make though, I worried myself sick because there is always a risk with anaesthetic. After the op, Flo had a black circle around her nose from the gas mask the dear little thing. She bounced back SO quickly though, the little madam was eating as soon as she woke up and a couple of weeks later when the fur grew back, we'd all pretty much forgotten she ever had a lump there!