Question about MRU and Padding in PPP

Hello there.

I am beginning to learn about PPP.
But I got stuck on MRU from start.
The following is the paragraph that make me crazy.
Please anybody explain me the meaning of the following paragraph.

==========================================================================
Page 73.

If the peer indicates a larger-thanexpected MRU, there is no harm in
sending
Configure-Ack for this value but then sending only the largest
messages possible,
even though these messages may be less than the requested MRU.
In other words, reasonable implementations should reply with
Configure-Ack
if the offered value is any value greater than some small lower bound
(say, 64 octets).
==========================================================================

"but then sending only the largest messages possible,
even though these messages may be less than the requested MRU. "

what I want to know is that when MRU is large, the ppp frame sender
should pad the PPP frame
to the size of received MRU.

I hope your good and easy answer.

thank you for reading my writing.