It's becoming more and more of a problem worldwide.
这成为世界越来越大的问题了!
become of = 成为
比如
The traffic in Beijing is becoming more of a challenge everyday.不加of行不行呢？是不是因为有more才必须加of？对， more of a 是习惯语，但口语也常说 more a challenge (of 是隐含词）。当然你也可以说The traffic in Beijing is becoming more challenging everyday. 异曲同工。说到底，这里的of还是和become无关啊对。和 become 无关。of 是跟 more 的。