Why the applet size is defined on html code instead of the java code?

Question

Is it because it is embedded on the website site and it is read before the java code? How is the applet size adjusted to its components size?

Answer 1

Is it because it is embedded on the website site

Yes

and it is read before the java code?

No

How is the applet size adjusted to its components size?

The applet adjusts the size and layout of its components. The webpage is responsible for sizing of the applet. The applet could expose data to the hosting app, which could be read via javascript.