Interdealer versus Interbank -- Spot FX

Kaplan notes:

The market (spot fx) works in three tiers: interdealer, interbank, and bank-to-client, with decreasing trade sizes and increasing spreads, respectively.”

However - in the Kaplan Qbank:

The interbank market has the largest trade sizes with the smallest spreads. The bank-to-client market has the smallest trades sizes with the largest spreads.”

Is Interbank or Interdealer the larger of the two markets in terms of what market handles the most size, and who has the lowest spread?

What does the curriculum say?

Curriculum states three levels in FX trades:
Interbank (major dealers) > second-tier banks and financial institutions > commercial companies and retails traders