MacBook Air Ventura user folder says 500 gb but contents are only 260

Did you copy large amounts of data to another location within the same APFS volume on the old Mac? If so, then that data was not physically copied.....only a small link was created to point to the original physical copy of the data. When you move all of that data to another drive, then you will end up with two physical copies of that data on the new drive. The original physical copy, then a 2nd copy of that physical copy associated with that small link I mentioned earlier. Unfortunately it is not possible to retain that APFS special copy when moving data to another drive/volume so you end up with double the used space for those particular items.

The new M4 MacBook Air would come preinstalled with macOS 15 Sequoia

The Internal SSD Drive of the new M4 MBA has a Capacity of 500 GB ?

One Size does Not fit All 

If the new computer is going to be the Apple Silicon  Computer - would suggest Only Migrating the User Account and nothing more.

Some have had success and usually they are well seasoned veterans who are well versed in trouble shooting 

Others have been left with a mess when Migrating everything.

Issues that may have existed on the older computer will be migrated to the New Computer.

This would badly pollute the new Original and Pristine installation of the new OS.

Applications, extensions and Software Drivers may or may not even work on the new computer.

IMHO - it would be best to only install new application etc Directly from the Developer or the Apple Apps Store.

Due Diligence on New Applications and make sure they are Sequoia compatible and factor in the ARMs CPU of the Apple Silicon Computer 

Lesson learned, the hard way, sorry to say

As for System Setting and inaccuracies.

It has never been a reliable source of information.

Yes, Disk Utilities is far more accurate

If you just purchased the M4 machine which was available March 11 I believe

You can Return it under the 14 Day Return Policy of Apple

Apple - Legal - Sales Policies - U.S. Retail Sales

Standard Return Policy

We believe you will be thrilled with the products you purchase from the Apple Store. In the unlikely event a product is not what you expected, you can return or exchange it with a receipt within 14 days of the date you receive the product from Apple. The product must be in its original condition with all of its included parts, accessories, and packaging and we’ll exchange it or offer a refund based on the payment method. In addition,