EBCDIC is a format with an 8 bit encoding. The basic EBCDIC character set has plenty of unused space, so not all the potential of the 8 bits is used. But it has codepages for handling non-English languages, such as EBCDIC 284 for latin America.
UTF-8 on the other hand is a multibyte encoding. The English character set is encoded on 7 useful butes. Other unicode characters could be encoded in up to 4 bytes.
The first thought that comes to mind, would be that the UTF8 uses multibyte and this could explain the difference in compression.
The conversion between EBCDIC codepages and UNICODE code map to the 256 first characters. This means that ecoding an EBCDIC file with a local code page, would use a couple of multibyte characters in UTF-8, but each mutlibyte would be 2 bytes long.
Your data shows that UTF-8 encoding is 236MB and EBCDIC encoding is 235MB. This means that at most 1M characters use a multibyte encoding in UTF8 (exactly 1MB if only 2 bytes, less if more bytes are needed). And 234MB use an English character set. So the huge difference in compression (60%) cannot be explained by just because of some multibyte chars.
Furthermore, compression algorithms tend to statistically find out and eliminate redundancies. They should compress the data to the minimum number of bits. The useful information in both files being the same, independently of the encoding, the difference should be minimal.
By deduction, the different encoding cannot explain a huge difference in compression, if the optimal algorithm was used.
According to wikipedia, ZIP files may use the following compression schemes:
Store (no compression), Shrink, Reduce (levels 1-4), Implode, Deflate,
Deflate64, bzip2, LZMA (EFS), WavPack, and PPMd
For performance reason, ZIP does not try all of these compressions to determine the absolutely best. It uses some heuristics to chose the most suitable algorithm. The very different statistical distribution of EBCDIC and UTF8 could explain that the heuristic choses different algorithms.
So, with the given information, it seems that ZIP uses a suboptimal encoding scheme, and not necessarily the same for both files.
But this assumption needs an experimental verification. So please check the compression algorithm used in the ZIP files (as explained here on SO). If it's different you have the reason. If it's the same, please update your question and tell us which one so that we can re-investigate with more relevant information.