Depending upon your Mail server and your iPad Mail settings, you may be using POP3 or IMAP. This will have considerable influence upon what you see on your iPad and iPhone - and where your Mail messages actually reside.
When using IMAP, your device(s) will download a copy of your email messages when fetched from your Mail providers email server. The Mail server will keep the original email messages until such time as they are proactively deleted from either your iPad or iPhone.
If instead you are using POP3, email clients can be configured to (a) download and leave a copy on the server - or (b) download and delete from the server. As such, email messages may be deleted from the Mail Server as soon as they are downloaded to your device(s).
If supported by your Mail provider, it is also possible to configure a mixed environment where some devices are using IMAP and others POP3. Doing so requires considerable care - as misconfiguration can lead to unexpected behaviour. IMAP is the more modern Mail protocol that, for most domestic Users, is the preferred option.
Your description suggests that your iPhone has a cached local copy of your email messages - whilst your iPad does not. Before drawing conclusion as to the actual problem, you would be well advised to access your Mail using your Mail Provider’s web portal (using a web browser) and verify whether or not the messages that appear to be missing from your iPad (but are present on the iPhone) are still available to view on the server.
