There must be other users of the OP's combo of film and developer so I am surprised that no user of this combo has replied and given an example of what a 10x15cm print looks like
There might be better combo's, agitation etc but I'd be surprised if the problem is down to just this combo of film and developer.
Let's face it if the scan of a 10x15 print is accurate and he says it is and if he can easily see grain with his naked eye on a 10x15cm print and he says he can, then this has to be the worst combo of film and developer I have ever seen
A pity that we don't have an example of a 10x15 print from FP4+ and dilution B of HC110
pentaxuser
Back around 1986 I tested FP4 in HC-110 and it was reasonably fine grained, the film has been improved since then

It does tend to be a touch lower EI in HC-110m than D76/ID-11.
This is a 10x15cm print, so approx 6"x4" and at that size you should not be able to see any grain with a film like FP4_ or HP5+. I've an HP5+ print in front of me slightly larger degree of enlargement an the grain isn't visible to the naked eye.
I'd put this down to craft, the ability to exert total control over the process cycle. If you give a few photographers similar cameras, film, developer, stop & fixer some will consistently produce excellent results, fine grain, excellent tones & sharpness, others will end up with grainier images, less tonality etc.
All that differs is the attention to detail through out the whole exposure and development cycle, consistent agitation, close regulation of temperatures.
Ian