All of these things would have to be done by Congress. The President is really not the dictator that the internet thinks he is (outside of some particular domains). But just to go through those:

Make it expensive to change the weight of a product.

How? Make the government track the size of ever possible consumable product and mandate a fee when changed? Beyond the enormous logistical effort for no obvious purpose, this would also make it costly for a company to add more product. Perhaps you only apply the fee when a size decreases, but then,, how do you handle the case where a company intentionally launches a smaller sized version for a different market, eg individual or snack sized portions? What if they launch a new size and then discontinue the older, larger one, so it technically didn't change? Does that have a fine? Sure, you can try to track all of this stuff carefully and determine what the net effect is, but that costs time and money all for no significant benefit.

Standardize the size and weight of a given type of product

Who determines the standard, and why? Why should it be illegal to sell a smaller or larger bag of chips or soda?

Require the packaging to alert consumers that the weight have changed in the last year and how much it has changed

This would just be one more tiny disclaimer line on the back that nobody would read. Not to mention, the size and weight is already on the package. Consumers are already perfectly capable of seeing the weight and deciding if the value for that price is good. I somewhat doubt most people would actually change their behavior by learning that there were ten more chips in the bag a year ago, and at any rate, companies know that consumers would rather pay the same price for less than pay a higher price for the same amount.

Tie the trademark of a given product to a certain weight.

That is categorically not how trademarks work.