Some spots at the Mentawai are getting crowded, namely at the famous Playground area, because access to this area became easier recently.
A couple of other particular spots at the Mentawai can also get crowded occasionally (Lance’s right and Maccaronis), but the majority – away from the Playground area – has no access neither accommodation available, and therefore it’s not crowded.
Not to talk about the other Sumatran archipelagos – Telo, Hinako, Banyak, etc. – that seem to have been forgotten by surf boats and resorts operators. Most of these islands are absolutely desert.
Remember that most of the other Indonesian surfing islands (Bali, Java, Lombok, Sumbawa, etc.) are way more crowded than the Mentawai because the accesses are easy and there’s lots of cheap accommodation there.
In short, the only crowded spots at the Mentawai are at the Playground area (northern Mentawai) …and we don’t go there very often (unless you want to).