In general, the greater the weight, the lower the relative premium.
Bars and coins come in a variety of sizes and weights. But that doesn't mean you should buy the heaviest bar you can afford, because...
When buying the bar or coin, you should also think about how you want to sell it again and what you want to use it for. Large bars (or coins) are less divisible. When you sell a large bar, you liquidate a significant investment. It is more practical to sell small bars or coins and the exact amount gold or silver for the exact amount of euros you need. Smaller coins and bars are therefore practical as a means of exchange and payment and as part of an emergency kit. The CombiBar is a good example where, for a relatively small additional cost, a bar can be made into fractions that are very useful as a means of payment in unforeseen times.
Read more in the Starter's Guide – What do I buy??