Hi,
I get the same result, just 19 sizes available. I believe the API gives outdated list of available droplet sizes. The sizes differs to the plans available through DO control panel during droplet creation process.
What may be interesting, I successfully created a droplet with the size that does not have corresponding plan. Here is the response I got after creation:

{"droplet":{"id":123456789,"name":"test-ubuntu2004","memory":1024,"vcpus":1,"disk":30,"locked":false,"status":"new","kernel":null,"created_at":"2020-10-10T19:32:21Z","features":[],"backup_ids":[],"next_backup_window":null,"snapshot_ids":[],"image":{"id":69463186,"name":"20.04 (LTS) x64","distribution":"Ubuntu","slug":"ubuntu-20-04-x64","public":true,"regions":["nyc3","nyc1","sfo1","nyc2","ams2","sgp1","lon1","ams3","fra1","tor1","sfo2","blr1","sfo3"],"created_at":"2020-09-03T02:05:57Z","min_disk_size":15,"type":"base","size_gigabytes":2.36,"description":"Ubuntu 20.04 x86","tags":[],"status":"available"},"volume_ids":[],"size":{"slug":"1gb","memory":1024,"vcpus":1,"disk":30,"transfer":2.0,"price_monthly":10.0,"price_hourly":0.01488,"regions":["ams2","ams3","blr1","fra1","lon1","nyc1","nyc2","nyc3","sfo1","sfo2","sfo3","sgp1","tor1"],"available":true},"size_slug":"1gb","networks":{"v4":[],"v6":[]},"region":{"name":"London 1","slug":"lon1","features":["private_networking","backups","ipv6","metadata","install_agent","storage","image_transfer"],"available":true,"sizes":["s-1vcpu-1gb","512mb","s-1vcpu-2gb","1gb","s-3vcpu-1gb","s-2vcpu-2gb","s-1vcpu-3gb","s-2vcpu-4gb","2gb","s-4vcpu-8gb","m-1vcpu-8gb","c-2","4gb","c2-2vcpu-4gb","g-2vcpu-8gb","gd-2vcpu-8gb","m-16gb","s-8vcpu-16gb","s-6vcpu-16gb","c-4","8gb","c2-4vpcu-8gb","m-2vcpu-16gb","m3-2vcpu-16gb","g-4vcpu-16gb","gd-4vcpu-16gb","m6-2vcpu-16gb","m-32gb","s-8vcpu-32gb","c-8","16gb","c2-8vpcu-16gb","m-4vcpu-32gb","m3-4vcpu-32gb","g-8vcpu-32gb","s-12vcpu-48gb","gd-8vcpu-32gb","m6-4vcpu-32gb","m-64gb","s-16vcpu-64gb","c-16","32gb","c2-16vcpu-32gb","m-8vcpu-64gb","m3-8vcpu-64gb","s-20vcpu-96gb","48gb","m6-8vcpu-64gb","m-128gb","s-24vcpu-128gb","64gb","c2-32vpcu-64gb","m-16vcpu-128gb","m3-16vcpu-128gb","s-32vcpu-192gb","m-24vcpu-192gb","m-224gb","m6-16vcpu-128gb","m3-24vcpu-192gb","m6-24vcpu-192gb","m3-32vcpu-256gb","m6-32vcpu-256gb"]},"tags":["test"]},"links":{"actions":[{"id":1041287508,"rel":"create","href":"https://api.digitalocean.com/v2/actions/1041287508"}]}}