There is not a great deal you can do about it, unless they were to release a firmware that allowed you to alter the sizes of the fonts you are going to be stuck with what you have.
However what you are seeing is a typical scenario, for instance SPA phones connected to an Asterisk system will have a SIP based firmware on it, and the fonts on this firmware are vastly different to that of the Skinny SCCP firmware that the SPA series phone has when plugged into a UC, the reason for this I would imaging be related to the functionality of the softkeys that are used to carry out functions, Asterisk will not utilize all that the UC does and thus they need to make sure that there is sufficient room on the screen to cater for everything.
You can try and petition on here for them to make the phones more flexible, however don't pin your hopes on it as I would imaging there are other items on the agenda that carry more weight and priority for future releases, even though I personally consider this as an important request as well
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