I thought I would add a few thoughts in addition to Alex.
The standard YCSB E workload is 5% insert operations and 95% range scans. Typically the range scans are for up to 100 records, but it sounds like you are experimenting with larger ranges. The record size is 1KB.
While it's great to hear that WiredTiger is showing good performance in your tests, it's possible that the performance you are seeing reflects other bottlenecks or optimizations in the system. Two items that come to mind are both cache related.
First, YSCB uses a Zipfian request distribution. This means the requests are heavily skewed towards a small number of records. So even with a constrained cache, your workload may have a good cache hit rate.
Second, even though you are constraining WiredTiger's cache, the test files probably fit in the operating system cache. So the performance penalty when you miss in the WiredTiger cache is will be small. In the extreme, if the WiredTiger cache is too small to help, you may essentially be measuring the performance of reading everything from the OS cache.
It's hard to provide a definitive answer to why a specific workload performs in a certain way because it depends on the workload, the system, configuration parameters etc.