Why does Mrs. Higgins tell Henry not to come to her social hour? What does she mean by his large talk?

Mrs.
Higgins tells Henry not to come to her social hour because he invariably ends up offending all
of her friends when he meets them. She also complains to Henry that her friends eventually stop
coming to her at-home days after making his acquaintance.

Shaw describes
Henry as the 'energetic, scientific type, heartily, even violently interested in everything that
can be studied as a scientific subject, and careless about himself and other
people, including their feelings.' In other words, Henry isn't very good at small
talk during social events; his manner generally varies from 'genial bullying when he is in a
good humor to stormy petulance when anything goes wrong.'

Henry is so fixated
and obsessed with his social experiments that he lacks the ability to endear himself to others;
in other words, he lacks the necessary patience to engage in polite conversation. This is
because he sees such conversation as fruitless and frivolous, serving no purpose
whatsoever. 

When Henry admits to his mother that he isn't very good at small
talk, she questions if his 'large talk' is any better. Henry's 'large talk' refers to the way he
speaks: he is either embarrassingly crude to the point of rudeness, or he resorts to speaking in
a highly stylized, elocutionary style when he wants to patronize and insult his hearers. In
colloquial English, we would say that Henry 'talks down' to people.

For example:

Higgins: Take her away and
clean her, Mrs. Pearce. Monkey Brand, if it won't come off any other way. (Here,
Henry rudely implies that Liza is extremely dirty and needs to be scrubbed
properly).

Higgins:Take all her clothes off and burn them.
(more rudeness).

Higgins:[with professional
exquisiteness of modulation] I walk over everybody! My dear Mrs. Pearce, my dear Pickering, I
never had the slightest intention of walking over anyone. All I propose is that we should be
kind to this poor girl. We must help her to prepare and fit herself for her new station in life.
If I did not express myself clearly it was because I did not wish to hurt her delicacy, or
yours. (Here, Henry shows that he is a master of the polite insult. His words and
manner are patronizing and hardly endears anyone to him).

HIGGINS [suddenly resorting to the most thrillingly beautiful low tones in
his best elocutionary style] By George, Eliza, the streets will be strewn with the
bodies of men shooting themselves for your sake before I've done with you. (Here,
Henry exhibits his arrogant belief that his own skills are so superior that he can transform
Liza into an exquisite lady any eligible man would kill to
marry).