For the purposes of this post, I will use the following definitions of the tiers.
- A tier 1 ISP is one that relies purely on peering and does not use transit.
- A tier 2 network (may or may not be an ISP) is one that uses a mixture of peering and transit.
- A tier 3 network (may or may not be an ISP) is one that relies purely on transit.
(many definitions of "teir 1" also require that the peering be settlement free, but that is not relevant to the issue at hand here and is not possible to reliably determine from outside)
You can't go pointing a default route at a peer, at least not if you want them to remain a peer in good standing. So the core routers of a tier 1 ISP will need to be default-free. To reach the whole internet without a default route, a full internet routing table is needed. Therefore the routers in the core network of a tier 1 ISP will need to carry the whole internet routing table.
A tier 2 or tier 3 network on the other hand can choose if they wish to filter the routing tables from their upstreams and peers and use a default route pointing at one of their transit providers for the residual traffic. This saves them router resources, but has the downside that if their upstream provider gets in a peering dispute they may lose access to parts of the Internet.
Many ISPs will split their network into "core" and "access" portions. The "core" side will have a full internet routing table, while the access side will have default routes pointing at the core side.