It's difficult to find a proper comparison, because we rarely have very many good records in terms of accounting from the period, so largely we need to go on what exactly they were capable of. In this case, defenses; Alfred was so strained that the only thing he could do to secure his borders was give peasants land in exchange for them building burhs. That's a bad fiscal state. A smaller country can have a better fiscal state and essentially be 'rich', even if they're making less money than a larger kingdom.