Thicker chains are generally more secure than thinner ones due to several factors. Firstly, the increased diameter of the links in a thicker chain provides greater resistance to cutting tools. Tools like bolt cutters, hacksaws, or angle grinders require more effort and time to penetrate thicker metal, thereby enhancing security. 
Secondly, thicker chains are typically made from stronger materials or alloys, which contribute to their overall durability and resistance to various forms of attack, including attempts to break or deform the chain. This makes them more suitable for high-security applications, such as securing valuable assets or in environments where theft is a significant risk.
Additionally, the weight of a thicker chain can act as a deterrent. Heavier chains are more cumbersome to handle and transport, making them less appealing to potential thieves who may seek a quick and easy target. 
However, the security of a chain is not solely dependent on its thickness. The quality of the material, the design of the links, and the integrity of the locking mechanism also play crucial roles. A well-designed thinner chain with a high-quality lock may offer better security than a poorly constructed thicker chain.
In summary, while thicker chains generally offer more security due to their resistance to cutting and breaking, the overall security is a combination of thickness, material quality, design, and the locking system used.