First, Ladybird is basically a one-man project and I don't think they are really looking to become a company as such. This means there is a practical maximum you can invest even if it was a very important project. The effect of investing 500k would be basically the same as investing 100k (ie, de dev behind can now work on the project full-time) and if you invest too much that might make it a shopify-exclusive project. If the goal is to sponsor a new widely used open source browser it makes sense not to monopolize it too much, because then people wouldn't see the project as "open" anymore. Now there can still be other sponsors.

Second, a project like this probably has a fairly low chance of actually succeeding in gaining market share from one of the big browsers. If shopify actually has some type of Strategic Investment Budget for commoditizing complements and the like, it would probably make sense to split it VC-style over many small projects rather than making big bets on only a few projects. If one of the small projects gains traction, you can always support it more when needed.