Might you have "Text Size" set to a larger than normal size and/or "Display Zoom" set to "Zoomed" (both of those settings are found under "Settings" / "Display and Brightness"). There are also settings under "Settings" / "General" / "Accessibility" (for example "Large text") that might have a bearing.
I've just had a play on my 6s - I was able to get the lock screen font somewhat larger than normal, but not to exceed the screen width (although I didn't try combinations of more than one of those settings)
[apologies - just reread your question and accept that you are deliberately using a larger font - there are programming tweaks that could have been used to autoshrink the font to fit, but the only other things to do would have been to abbreviate the month or allow the date to wrap]