There are actually two different laws for two different people. The Talmud in several places (e.g. Berachot 5b) lays down the following law:
אחד המרבה ואחד הממעיט ובלבד שיכוין לבו לשמים
One person must do a lot and one person must do a little, but both have to do it for the sake of Heaven.
Thus the statement that you quote regarding not going back to speak more is referring to the person whose job it is to speak only a little. The Sages are thus quantifying "a little" as one time – once he speaks he cannot go back and speak again.
The person whose job it is to speak a lot can speak as many times as he wants. However, there are certain criteria pertaining to who should be the אחד המרבה and who should be the אחד הממעיט. Exodus 30:15 tells us that the מרבה should be poor and the ממעיט should be rich:
העשיר לא ירבה והדל לא ימעיט
Other criteria include that the מרבה must not be an animal and must not be a woman, based on Deuteronomy 17:16 (לא ירבה לו סוסים) and 17:17 (לא ירבה לו נשים).
The statement that you quote from the Haggada should be read as follows:
וכל המרבה לספר ביציאת מצרים הרי זה משובח
Whoever is the מרבה [i.e. the one whose job it is to speak a lot] for speaking about leaving Egypt is praiseworthy.
This statement is telling us that even though it is necessary to have someone speak a lot and someone speak a little, it is better to be the person who speaks a lot. This is in line with the Beraita cited in Kiddushin 82b which notes that it is necessary for all positions to be filled, but some positions are still more praiseworthy than others:
אי אפשר לעולם בלא בסם ובלא בורסקי אשרי מי שאומנותו בסם ואוי לו מי שאומנותו בורסקי
It is impossible for the world [to be] without a perfumer and without a tanner. [But] fortunate is he whose profession is perfuming and woe to he whose profession is tanning.
Of course, this raises the question of why it is better to be the מרבה than to be the ממעיט. This is a bit complex, but here goes. We know from Ecclesiastes 10:14 that the מרבה is a fool:
והסכל ירבה דברים
The מרבה being a fool causes a problem for the ממעיט. When the מרבה speaks the ממעיט cannot respond (if he has already spoken than he can't respond because of שוב אינו חוזר, and if he has not already spoken then he can't respond because Proverbs 26:4 forbids responding to a fool (אל תען כסיל כאולתו)). However, if the ממעיט does not respond, he will be in violation of Proverbs 26:5 which says that one should respond to a fool (ענה כסיל כאולתו). Thus, by speaking, the מרבה invariably causes the ממעיט to sin.
That the מרבה causes sin is elsewhere attested to by the Sages. In Avot 1:17 we find this stated explicitly:
וכל המרבה דברים מביא חטא
Now we have a principle mentioned in Bava Batra 9a which states:
גדול המעשה יותר מן העושה
Greater is he who causes it to be done than he who does it.
In our case the ממעיט is the one sinning and the מרבה is the one causing him to sin. It follows, then, that the מרבה is greater than the ממעיט; hence the Sages said that it is praiseworthy to be the מרבה.